Add Camera::viewport_to_world_2d (bevyengine#6557)

# Objective

Add a simpler and less expensive 2D variant of `viewport_to_world`.

Co-authored-by: devil-ira <justthecooldude@gmail.com>

crates/bevy_render/src/camera/camera.rs

@@ -248,6 +248,25 @@ impl Camera {
         })
     }
 
+    /// Returns a 2D world position computed from a position on this [`Camera`]'s viewport.
+    ///
+    /// Useful for 2D cameras and other cameras with an orthographic projection pointing along the Z axis.
+    ///
+    /// To get the world space coordinates with Normalized Device Coordinates, you should use
+    /// [`ndc_to_world`](Self::ndc_to_world).
+    pub fn viewport_to_world_2d(
+        &self,
+        camera_transform: &GlobalTransform,
+        viewport_position: Vec2,
+    ) -> Option<Vec2> {
+        let target_size = self.logical_viewport_size()?;
+        let ndc = viewport_position * 2. / target_size - Vec2::ONE;
+
+        let world_near_plane = self.ndc_to_world(camera_transform, ndc.extend(1.))?;
+
+        Some(world_near_plane.truncate())
+    }
+
     /// Given a position in world space, use the camera's viewport to compute the Normalized Device Coordinates.
     ///
     /// When the position is within the viewport the values returned will be between -1.0 and 1.0 on the X and Y axes,