Fix a system ordering issue with motion blur for skinned meshes. (#15693

)

Currently, it's possible for the `collect_meshes_for_gpu_building`
system to run after `set_mesh_motion_vector_flags`. This will cause
those motion vector flags to be overwritten, which will cause the shader
to ignore the motion vectors for skinned meshes, which will cause
graphical artifacts.

This patch corrects the issue by forcing `set_mesh_motion_vector_flags`
to run after `collect_meshes_for_gpu_building`.

crates/bevy_pbr/src/render/mesh.rs

@@ -163,7 +163,7 @@ impl Plugin for MeshRenderPlugin {
  .add_systems(
  Render,
  (
- set_mesh_motion_vector_flags.before(RenderSet::Queue),
+ set_mesh_motion_vector_flags.in_set(RenderSet::PrepareAssets),
  prepare_skins.in_set(RenderSet::PrepareResources),
  prepare_morphs.in_set(RenderSet::PrepareResources),
  prepare_mesh_bind_group.in_set(RenderSet::PrepareBindGroups),
@@ -208,7 +208,11 @@ impl Plugin for MeshRenderPlugin {
  .after(prepare_view_targets),
  collect_meshes_for_gpu_building
  .in_set(RenderSet::PrepareAssets)
- .after(allocator::allocate_and_free_meshes),
+ .after(allocator::allocate_and_free_meshes)
+ // This must be before
+ // `set_mesh_motion_vector_flags` so it doesn't
+ // overwrite those flags.
+ .before(set_mesh_motion_vector_flags),
  ),
  );
  } else {