Recalculate entity aabbs when meshes change (bevyengine#4944)

# Objective

Update the `calculate_bounds` system to update `Aabb`s
for entities who've either:
- gotten a new mesh
- had their mesh mutated

Fixes bevyengine#4294.

## Solution

There are two commits here to address the two issues above:

### Commit 1

**This Commit**
Updates the `calculate_bounds` system to operate not only on entities
without `Aabb`s but also on entities whose `Handle<Mesh>` has changed.

**Why?**
So if an entity gets a new mesh, its associated `Aabb` is properly
recalculated.

**Questions**
- This type is getting pretty gnarly - should I extract some types?
- This system is public - should I add some quick docs while I'm here?

### Commit 2

**This Commit**
Updates `calculate_bounds` to update `Aabb`s of entities whose meshes
have been directly mutated.

**Why?**
So if an entity's mesh gets updated, its associated `Aabb` is properly
recalculated.

**Questions**
- I think we should be using `ahash`. Do we want to do that with a
  direct `hashbrown` dependency or an `ahash` dependency that we
  configure the `HashMap` with?
- There is an edge case of duplicates with `Vec<Entity>` in the
  `HashMap`. If an entity gets its mesh handle changed and changed back
  again it'll be added to the list twice. Do we want to use a `HashSet`
  to avoid that? Or do a check in the list first (assuming iterating
  over the `Vec` is faster and this edge case is rare)?
- There is an edge case where, if an entity gets a new mesh handle and
  then its old mesh is updated, we'll update the entity's `Aabb` to the
  new geometry of the _old_ mesh. Do we want to remove items from the
  `Local<HashMap>` when handles change? Does the `Changed` event give us
  the old mesh handle? If not we might need to have a
  `HashMap<Entity, Handle<Mesh>>` or something so we can unlink entities
  from mesh handles when the handle changes.
- I did the `zip()` with the two `HashMap` gets assuming those would
  be faster than calculating the Aabb of the mesh (otherwise we could do
  `meshes.get(mesh_handle).and_then(Mesh::compute_aabb).zip(entity_mesh_map...)`
  or something). Is that assumption way off?

## Testing

I originally tried testing this with `bevy_mod_raycast` as mentioned in the
original issue but it seemed to work (maybe they are currently manually
updating the Aabbs?). I then tried doing it in 2D but it looks like
`Handle<Mesh>` is just for 3D. So I took [this example](https://github.com/bevyengine/bevy/blob/main/examples/3d/pbr.rs)
and added some systems to mutate/assign meshes:

<details>
<summary>Test Script</summary>

```rust
use bevy::prelude::*;
use bevy::render::camera::ScalingMode;
use bevy::render::primitives::Aabb;

/// Make sure we only mutate one mesh once.
#[derive(Eq, PartialEq, Clone, Debug, Default)]
struct MutateMeshState(bool);

/// Let's have a few global meshes that we can cycle between.
/// This way we can be assigned a new mesh, mutate the old one, and then get the old one assigned.
#[derive(Eq, PartialEq, Clone, Debug, Default)]
struct Meshes(Vec<Handle<Mesh>>);

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .init_resource::<MutateMeshState>()
        .init_resource::<Meshes>()
        .add_startup_system(setup)
        .add_system(assign_new_mesh)
        .add_system(show_aabbs.after(assign_new_mesh))
        .add_system(mutate_meshes.after(show_aabbs))
        .run();
}

fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut global_meshes: ResMut<Meshes>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) {
    let m1 = meshes.add(Mesh::from(shape::Icosphere::default()));
    let m2 = meshes.add(Mesh::from(shape::Icosphere {
        radius: 0.90,
        ..Default::default()
    }));
    let m3 = meshes.add(Mesh::from(shape::Icosphere {
        radius: 0.80,
        ..Default::default()
    }));
    global_meshes.0.push(m1.clone());
    global_meshes.0.push(m2);
    global_meshes.0.push(m3);

    // add entities to the world
    // sphere
    commands.spawn_bundle(PbrBundle {
        mesh: m1,
        material: materials.add(StandardMaterial {
            base_color: Color::hex("ffd891").unwrap(),
            ..default()
        }),
        ..default()
    });
    // new 3d camera
    commands.spawn_bundle(Camera3dBundle {
        projection: OrthographicProjection {
            scale: 3.0,
            scaling_mode: ScalingMode::FixedVertical(1.0),
            ..default()
        }
        .into(),
        ..default()
    });

    // old 3d camera
    // commands.spawn_bundle(OrthographicCameraBundle {
    //     transform: Transform::from_xyz(0.0, 0.0, 8.0).looking_at(Vec3::default(), Vec3::Y),
    //     orthographic_projection: OrthographicProjection {
    //         scale: 0.01,
    //         ..default()
    //     },
    //     ..OrthographicCameraBundle::new_3d()
    // });
}
fn show_aabbs(query: Query<(Entity, &Handle<Mesh>, &Aabb)>) {
    for thing in query.iter() {
        println!("{thing:?}");
    }
}

/// For testing the second part - mutating a mesh.
///
/// Without the fix we should see this mutate an old mesh and it affects the new mesh that the
/// entity currently has.
/// With the fix, the mutation doesn't affect anything until the entity is reassigned the old mesh.
fn mutate_meshes(
    mut meshes: ResMut<Assets<Mesh>>,
    time: Res<Time>,
    global_meshes: Res<Meshes>,
    mut mutate_mesh_state: ResMut<MutateMeshState>,
) {
    let mutated = mutate_mesh_state.0;
    if time.seconds_since_startup() > 4.5 && !mutated {
        println!("Mutating {:?}", global_meshes.0[0]);
        let m = meshes.get_mut(&global_meshes.0[0]).unwrap();
        let mut p = m.attribute(Mesh::ATTRIBUTE_POSITION).unwrap().clone();
        use bevy::render::mesh::VertexAttributeValues;
        match &mut p {
            VertexAttributeValues::Float32x3(v) => {
                v[0] = [10.0, 10.0, 10.0];
            }
            _ => unreachable!(),
        }
        m.insert_attribute(Mesh::ATTRIBUTE_POSITION, p);
        mutate_mesh_state.0 = true;
    }
}

/// For testing the first part - assigning a new handle.
fn assign_new_mesh(
    mut query: Query<&mut Handle<Mesh>, With<Aabb>>,
    time: Res<Time>,
    global_meshes: Res<Meshes>,
) {
    let s = time.seconds_since_startup() as usize;
    let idx = s % global_meshes.0.len();
    for mut handle in query.iter_mut() {
        *handle = global_meshes.0[idx].clone_weak();
    }
}
```

</details>

## Changelog

### Fixed
Entity `Aabb`s not updating when meshes are mutated or re-assigned.

crates/bevy_render/src/view/visibility/mod.rs

@@ -1,15 +1,18 @@
 mod render_layers;
 
+use smallvec::SmallVec;
+
 pub use render_layers::*;
 
 use bevy_app::{CoreStage, Plugin};
-use bevy_asset::{Assets, Handle};
+use bevy_asset::{AssetEvent, Assets, Handle};
 use bevy_ecs::prelude::*;
 use bevy_hierarchy::{Children, Parent};
 use bevy_reflect::std_traits::ReflectDefault;
 use bevy_reflect::Reflect;
 use bevy_transform::components::GlobalTransform;
 use bevy_transform::TransformSystem;
+use bevy_utils::HashMap;
 use std::cell::Cell;
 use thread_local::ThreadLocal;
 
@@ -127,6 +130,11 @@ pub struct VisibilityBundle {
 #[derive(Component)]
 pub struct NoFrustumCulling;
 
+/// Use this component to opt-out of built-in [`Aabb`] updating for Mesh entities (i.e. to opt out
+/// of [`update_bounds`]).
+#[derive(Component)]
+pub struct NoAabbUpdate;
+
 /// Collection of entities visible from the current view.
 ///
 /// This component contains all entities which are visible from the currently
@@ -160,9 +168,59 @@ impl VisibleEntities {
     }
 }
 
+/// Tracks which [`Entities`](Entity) have which meshes for entities whose [`Aabb`]s are managed by
+/// the [`calculate_bounds`] and [`update_bounds`] systems. This is needed because `update_bounds`
+/// recomputes `Aabb`s for entities whose mesh has been mutated. These mutations are visible via
+/// [`AssetEvent<Mesh>`](AssetEvent) which tells us which mesh was changed but not which entities
+/// have that mesh.
+#[derive(Debug, Default, Clone)]
+pub struct EntityMeshMap {
+    entities_with_mesh: HashMap<Handle<Mesh>, SmallVec<[Entity; 1]>>,
+    mesh_for_entity: HashMap<Entity, Handle<Mesh>>,
+}
+
+impl EntityMeshMap {
+    /// Register the passed `entity` as having the passed `mesh_handle`.
+    fn register(&mut self, entity: Entity, mesh_handle: &Handle<Mesh>) {
+        // Note that this list can have duplicates if an entity is registered for a mesh multiple
+        // times. This should be rare and only cause an additional `Aabb.clone()` in
+        // `update_bounds` so it is preferable to a `HashSet` for now.
+        self.entities_with_mesh
+            .entry(mesh_handle.clone_weak())
+            .or_default()
+            .push(entity);
+        self.mesh_for_entity
+            .insert(entity, mesh_handle.clone_weak());
+    }
+
+    /// Deregisters the mapping between an `Entity` and `Mesh`. Used so [`update_bounds`] can
+    /// track which mappings are still active so `Aabb`s are updated correctly.
+    fn deregister(&mut self, entity: Entity) {
+        let mut inner = || {
+            let mesh = self.mesh_for_entity.remove(&entity)?;
+
+            // This lookup failing is _probably_ an error.
+            let entities = self.entities_with_mesh.get_mut(&mesh)?;
+
+            // There could be duplicate entries in here if an entity was registered with a mesh
+            // multiple times. It's important to remove all references so that if an entity gets a
+            // new mesh and its old mesh is mutated, the entity doesn't get its old mesh's new
+            // `Aabb`. Note that there _should_ only be one entity.
+            for i in (0..entities.len()).rev() {
+                if entities[i] == entity {
+                    entities.swap_remove(i);
+                }
+            }
+            Some(())
+        };
+        inner();
+    }
+}
+
 #[derive(Debug, Hash, PartialEq, Eq, Clone, SystemLabel)]
 pub enum VisibilitySystems {
     CalculateBounds,
+    UpdateBounds,
     UpdateOrthographicFrusta,
     UpdatePerspectiveFrusta,
     UpdateProjectionFrusta,
@@ -178,60 +236,121 @@ impl Plugin for VisibilityPlugin {
     fn build(&self, app: &mut bevy_app::App) {
         use VisibilitySystems::*;
 
-        app.add_system_to_stage(
-            CoreStage::PostUpdate,
-            calculate_bounds.label(CalculateBounds),
-        )
-        .add_system_to_stage(
-            CoreStage::PostUpdate,
-            update_frusta::<OrthographicProjection>
-                .label(UpdateOrthographicFrusta)
-                .after(TransformSystem::TransformPropagate),
-        )
-        .add_system_to_stage(
-            CoreStage::PostUpdate,
-            update_frusta::<PerspectiveProjection>
-                .label(UpdatePerspectiveFrusta)
-                .after(TransformSystem::TransformPropagate),
-        )
-        .add_system_to_stage(
-            CoreStage::PostUpdate,
-            update_frusta::<Projection>
-                .label(UpdateProjectionFrusta)
-                .after(TransformSystem::TransformPropagate),
-        )
-        .add_system_to_stage(
-            CoreStage::PostUpdate,
-            visibility_propagate_system.label(VisibilityPropagate),
-        )
-        .add_system_to_stage(
-            CoreStage::PostUpdate,
-            check_visibility
-                .label(CheckVisibility)
-                .after(CalculateBounds)
-                .after(UpdateOrthographicFrusta)
-                .after(UpdatePerspectiveFrusta)
-                .after(UpdateProjectionFrusta)
-                .after(VisibilityPropagate)
-                .after(TransformSystem::TransformPropagate),
-        );
+        app.init_resource::<EntityMeshMap>()
+            .add_system_to_stage(
+                CoreStage::PostUpdate,
+                calculate_bounds.label(CalculateBounds),
+            )
+            .add_system_to_stage(CoreStage::PostUpdate, update_bounds.label(UpdateBounds))
+            .add_system_to_stage(
+                CoreStage::PostUpdate,
+                update_frusta::<OrthographicProjection>
+                    .label(UpdateOrthographicFrusta)
+                    .after(TransformSystem::TransformPropagate),
+            )
+            .add_system_to_stage(
+                CoreStage::PostUpdate,
+                update_frusta::<PerspectiveProjection>
+                    .label(UpdatePerspectiveFrusta)
+                    .after(TransformSystem::TransformPropagate),
+            )
+            .add_system_to_stage(
+                CoreStage::PostUpdate,
+                update_frusta::<Projection>
+                    .label(UpdateProjectionFrusta)
+                    .after(TransformSystem::TransformPropagate),
+            )
+            .add_system_to_stage(
+                CoreStage::PostUpdate,
+                visibility_propagate_system.label(VisibilityPropagate),
+            )
+            .add_system_to_stage(
+                CoreStage::PostUpdate,
+                check_visibility
+                    .label(CheckVisibility)
+                    .after(CalculateBounds)
+                    .after(UpdateBounds)
+                    .after(UpdateOrthographicFrusta)
+                    .after(UpdatePerspectiveFrusta)
+                    .after(UpdateProjectionFrusta)
+                    .after(VisibilityPropagate)
+                    .after(TransformSystem::TransformPropagate),
+            );
     }
 }
 
+/// Calculates [`Aabb`]s for [`Entities`](Entity) with [`Mesh`]es.
 pub fn calculate_bounds(
     mut commands: Commands,
     meshes: Res<Assets<Mesh>>,
     without_aabb: Query<(Entity, &Handle<Mesh>), (Without<Aabb>, Without<NoFrustumCulling>)>,
+    mut entity_mesh_map: ResMut<EntityMeshMap>,
 ) {
     for (entity, mesh_handle) in &without_aabb {
         if let Some(mesh) = meshes.get(mesh_handle) {
             if let Some(aabb) = mesh.compute_aabb() {
+                entity_mesh_map.register(entity, mesh_handle);
                 commands.entity(entity).insert(aabb);
             }
         }
     }
 }
 
+/// Updates [`Aabb`]s for [`Entities`](Entity) with [`Mesh`]es. This includes `Entities` that have
+/// been assigned new `Mesh`es as well as `Entities` whose `Mesh` has been directly mutated.
+///
+/// To opt out of bound calculation for an `Entity`, give it the [`NoAabbUpdate`] component.
+///
+/// NOTE: This system needs to remove entities from their collection in
+/// [`EntityMeshMap`] whenever a mesh handle is reassigned or an entity's mesh handle is
+/// removed. This may impact performance if meshes with many entities are frequently
+/// reassigned/removed.
+pub fn update_bounds(
+    mut commands: Commands,
+    meshes: Res<Assets<Mesh>>,
+    mut mesh_reassigned: Query<
+        (Entity, &Handle<Mesh>, &mut Aabb),
+        (
+            Changed<Handle<Mesh>>,
+            Without<NoFrustumCulling>,
+            Without<NoAabbUpdate>,
+        ),
+    >,
+    mut entity_mesh_map: ResMut<EntityMeshMap>,
+    mut mesh_events: EventReader<AssetEvent<Mesh>>,
+    entities_lost_mesh: RemovedComponents<Handle<Mesh>>,
+) {
+    for entity in entities_lost_mesh.iter() {
+        entity_mesh_map.deregister(entity);
+    }
+
+    for (entity, mesh_handle, mut aabb) in mesh_reassigned.iter_mut() {
+        entity_mesh_map.deregister(entity);
+        if let Some(mesh) = meshes.get(mesh_handle) {
+            if let Some(new_aabb) = mesh.compute_aabb() {
+                entity_mesh_map.register(entity, mesh_handle);
+                *aabb = new_aabb;
+            }
+        }
+    }
+
+    let to_update = |event: &AssetEvent<Mesh>| {
+        let handle = match event {
+            AssetEvent::Modified { handle } => handle,
+            _ => return None,
+        };
+        let mesh = meshes.get(handle)?;
+        let entities_with_handle = entity_mesh_map.entities_with_mesh.get(handle)?;
+        let aabb = mesh.compute_aabb()?;
+        Some((aabb, entities_with_handle))
+    };
+    for (aabb, entities_with_handle) in mesh_events.iter().filter_map(to_update) {
+        for entity in entities_with_handle {
+            commands.entity(*entity).insert(aabb.clone());
+        }
+    }
+}
+
 pub fn update_frusta<T: Component + CameraProjection + Send + Sync + 'static>(
     mut views: Query<(&GlobalTransform, &T, &mut Frustum)>,
 ) {