Fix motion blur on skinned meshes (#18712)

## Objective

Fix motion blur not working on skinned meshes.

## Solution

`set_mesh_motion_vector_flags` can set
`RenderMeshInstanceFlags::HAS_PREVIOUS_SKIN` after specialization has
already cached the material. This can lead to
`MeshPipelineKey::HAS_PREVIOUS_SKIN` never getting set, disabling motion
blur.

The fix is to make sure `set_mesh_motion_vector_flags` happens before
specialization.

Note that the bug is fixed in a different way by #18074, which includes
other fixes but is a much larger change.

## Testing

Open the `animated_mesh` example and add these components to the
`Camera3d` entity:

```rust
MotionBlur {
    shutter_angle: 5.0,
    samples: 2,
    #[cfg(all(feature = "webgl2", target_arch = "wasm32", not(feature = "webgpu")))]
    _webgl2_padding: Default::default(),
},
#[cfg(all(feature = "webgl2", target_arch = "wasm32", not(feature = "webgpu")))]
Msaa::Off,
```

Tested on `animated_mesh`, `many_foxes`, `custom_skinned_mesh`,
Win10/Nvidia with Vulkan, WebGL/Chrome, WebGPU/Chrome. Note that testing
`many_foxes` WebGL requires #18715.

Author: greeble-dev

crates/bevy_pbr/src/material.rs

@@ -327,7 +327,8 @@ where
                             .in_set(RenderSet::PrepareMeshes)
                             .after(prepare_assets::<PreparedMaterial<M>>)
                             .after(prepare_assets::<RenderMesh>)
-                            .after(collect_meshes_for_gpu_building),
+                            .after(collect_meshes_for_gpu_building)
+                            .after(set_mesh_motion_vector_flags),
                         queue_material_meshes::<M>
                             .in_set(RenderSet::QueueMeshes)
                             .after(prepare_assets::<PreparedMaterial<M>>),

crates/bevy_pbr/src/prepass/mod.rs

@@ -3,11 +3,11 @@ mod prepass_bindings;
 use crate::{
     alpha_mode_pipeline_key, binding_arrays_are_usable, buffer_layout,
     collect_meshes_for_gpu_building, material_bind_groups::MaterialBindGroupAllocator,
-    queue_material_meshes, setup_morph_and_skinning_defs, skin, DrawMesh,
-    EntitySpecializationTicks, Material, MaterialPipeline, MaterialPipelineKey, MeshLayouts,
-    MeshPipeline, MeshPipelineKey, OpaqueRendererMethod, PreparedMaterial, RenderLightmaps,
-    RenderMaterialInstances, RenderMeshInstanceFlags, RenderMeshInstances, RenderPhaseType,
-    SetMaterialBindGroup, SetMeshBindGroup, ShadowView, StandardMaterial,
+    queue_material_meshes, set_mesh_motion_vector_flags, setup_morph_and_skinning_defs, skin,
+    DrawMesh, EntitySpecializationTicks, Material, MaterialPipeline, MaterialPipelineKey,
+    MeshLayouts, MeshPipeline, MeshPipelineKey, OpaqueRendererMethod, PreparedMaterial,
+    RenderLightmaps, RenderMaterialInstances, RenderMeshInstanceFlags, RenderMeshInstances,
+    RenderPhaseType, SetMaterialBindGroup, SetMeshBindGroup, ShadowView, StandardMaterial,
 };
 use bevy_app::{App, Plugin, PreUpdate};
 use bevy_render::{
@@ -219,7 +219,8 @@ where
                         .in_set(RenderSet::PrepareMeshes)
                         .after(prepare_assets::<PreparedMaterial<M>>)
                         .after(prepare_assets::<RenderMesh>)
-                        .after(collect_meshes_for_gpu_building),
+                        .after(collect_meshes_for_gpu_building)
+                        .after(set_mesh_motion_vector_flags),
                     queue_prepass_material_meshes::<M>
                         .in_set(RenderSet::QueueMeshes)
                         .after(prepare_assets::<PreparedMaterial<M>>)

crates/bevy_pbr/src/render/mesh.rs

@@ -1648,7 +1648,7 @@ fn extract_mesh_for_gpu_building(
 /// [`crate::material::queue_material_meshes`] check the skin and morph target
 /// tables for each mesh, but that would be too slow in the hot mesh queuing
 /// loop.
-fn set_mesh_motion_vector_flags(
+pub(crate) fn set_mesh_motion_vector_flags(
     mut render_mesh_instances: ResMut<RenderMeshInstances>,
     skin_uniforms: Res<SkinUniforms>,
     morph_indices: Res<MorphIndices>,