## 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.
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