Fix MSAA writeback when 3 or more cameras have the same target. (#11968)

# Objective

If multiple cameras render to the same target with MSAA enabled, only
the first and the last camera output will appear in the final output*.
This is because each camera maintains a separate flag to track the
active main texture. The first camera renders to texture A and all
subsequent cameras first write-back from A and then render into texture
B. Hence, camera 3 onwards will overwrite the work of the previous
camera.

\* This would manifest slightly differently if there were other calls to
post_process_write() in a more complex setup.

The is a functional regression from Bevy 0.12.

## Solution

The flag which tracks the active main texture should be shared between
cameras with the same `NormalizedRenderTarget`. Add the
`Arc<AtomicUsize>` to the existing per-target cache.

crates/bevy_render/src/view/mod.rs

@@ -498,7 +498,7 @@ pub fn prepare_view_targets(
                     _ => Some(clear_color_global.0),
                 };
 
-                let (a, b, sampled) = textures
+                let (a, b, sampled, main_texture) = textures
                     .entry((camera.target.clone(), view.hdr))
                     .or_insert_with(|| {
                         let descriptor = TextureDescriptor {
@@ -547,13 +547,14 @@ pub fn prepare_view_targets(
                         } else {
                             None
                         };
-                        (a, b, sampled)
+                        let main_texture = Arc::new(AtomicUsize::new(0));
+                        (a, b, sampled, main_texture)
                     });
 
                 let main_textures = MainTargetTextures {
                     a: ColorAttachment::new(a.clone(), sampled.clone(), clear_color),
                     b: ColorAttachment::new(b.clone(), sampled.clone(), clear_color),
-                    main_texture: Arc::new(AtomicUsize::new(0)),
+                    main_texture: main_texture.clone(),
                 };
 
                 commands.entity(entity).insert(ViewTarget {