Always update clusters and remove per-frame allocations (#4169)

 * Refactor assign_lights_to_clusters to always clear + update clusters, even if the screen size isn't available yet / is zero. This fixes #4167. We still avoid the "expensive" per-light work when the screen size isn't available yet. I also consolidated some logic to eliminate some redundancies.
* Removed _a ton_ of (potentially very large) per-frame reallocations
  * Removed `Res<VisiblePointLights>` (a vec) in favor of  `Res<GlobalVisiblePointLights>` (a hashmap). We were allocating a new hashmap every frame, the collecting it into a vec every frame, then in another system _re-generating the hashmap_. It is always used like a hashmap, might as well embrace that. We now reuse the same hashmap every frame and dont use any intermediate collections.
  * We were re-allocating Clusters aabb and light vectors every frame by re-constructing Clusters every frame. We now re-use the existing collections.
  * Reuse per-camera VisiblePointLight vecs when possible instead of allocating them every frame. We now only insert VisiblePointLights if the component doesn't exist yet.

crates/bevy_pbr/src/lib.rs

@@ -71,9 +71,9 @@ impl Plugin for PbrPlugin {
             .add_plugin(MeshRenderPlugin)
             .add_plugin(MaterialPlugin::<StandardMaterial>::default())
             .init_resource::<AmbientLight>()
+            .init_resource::<GlobalVisiblePointLights>()
             .init_resource::<DirectionalLightShadowMap>()
             .init_resource::<PointLightShadowMap>()
-            .init_resource::<VisiblePointLights>()
             .add_system_to_stage(
                 CoreStage::PostUpdate,
                 // NOTE: Clusters need to have been added before update_clusters is run so