Expose set_changed() on ResMut and Mut (#2208)

This new api stems from this [discord conversation](https://discord.com/channels/691052431525675048/742569353878437978/844057268172357663).

This exposes a public facing `set_changed` method on `ResMut` and `Mut`.

As a side note: `ResMut` and `Mut` have a lot of duplicated code, I have a PR I may put up later that refactors these commonalities into a trait.

Co-authored-by: Carter Anderson <mcanders1@gmail.com>

crates/bevy_ecs/src/system/system_param.rs

@@ -369,6 +369,16 @@ impl<'w, T: Component> ResMut<'w, T> {
         self.ticks
             .is_changed(self.last_change_tick, self.change_tick)
     }
+
+    /// Manually flags this resource as having been changed. This normally isn't
+    /// required because accessing this pointer mutably automatically flags this
+    /// resource as "changed".
+    ///
+    /// **Note**: This operation is irreversible.
+    #[inline]
+    pub fn set_changed(&mut self) {
+        self.ticks.set_changed(self.change_tick);
+    }
 }
 
 impl<'w, T: Component> Deref for ResMut<'w, T> {
@@ -381,7 +391,7 @@ impl<'w, T: Component> Deref for ResMut<'w, T> {
 
 impl<'w, T: Component> DerefMut for ResMut<'w, T> {
     fn deref_mut(&mut self) -> &mut Self::Target {
-        self.ticks.set_changed(self.change_tick);
+        self.set_changed();
         self.value
     }
 }

crates/bevy_ecs/src/world/pointer.rs

@@ -14,6 +14,16 @@ impl<'a, T> Mut<'a, T> {
         self.component_ticks.set_changed(self.change_tick);
         self.value
     }
+
+    /// Manually flags this value as having been changed. This normally isn't
+    /// required because accessing this pointer mutably automatically flags this
+    /// value as "changed".
+    ///
+    /// **Note**: This operation is irreversible.
+    #[inline]
+    pub fn set_changed(&mut self) {
+        self.component_ticks.set_changed(self.change_tick);
+    }
 }
 
 impl<'a, T> Deref for Mut<'a, T> {
@@ -28,7 +38,7 @@ impl<'a, T> Deref for Mut<'a, T> {
 impl<'a, T> DerefMut for Mut<'a, T> {
     #[inline]
     fn deref_mut(&mut self) -> &mut T {
-        self.component_ticks.set_changed(self.change_tick);
+        self.set_changed();
         self.value
     }
 }