fix shadow batching

[robtfm]

Objective

RenderMeshInstance::material_bind_group_id is only set from queue_material_meshes::<M>. this field is used (only) for determining batch groups, so some items may be batched incorrectly if they have never been in the camera's view or if they don't use the Material abstraction.

in particular, shadow views render more meshes than the main camera, and currently batch some meshes where the object has never entered the camera view together. this is quite hard to trigger, but should occur in a scene with out-of-view alpha-mask materials (so that the material instance actually affects the shadow) in the path of a light.

this is also a footgun for custom pipelines: failing to set the material_bind_group_id will result in all meshes being batched together and all using the closest/furthest material to the camera (depending on sort order).

Solution

• queue_shadows now sets the material_bind_group_id correctly
• MeshPipeline doesn't attempt to batch meshes if the material_bind_group_id has not been set. custom pipelines still need to set this field to take advantage of batching, but will at least render correctly if it is not set

[NthTensor]

Is there a shadow stress tests I should benchmark this against?

[robtfm]

This is about correctness rather than performance. The other pr mentioned above is about perf and includes a shadow feature in the many_cubes benchmark.