Currently, `RenderMaterialInstances` and `RenderMeshMaterialIds` are
very similar render-world resources: the former maps main world meshes
to typed material asset IDs, and the latter maps main world meshes to
untyped material asset IDs. This is needlessly-complex and wasteful, so
this patch unifies the two in favor of a single untyped
`RenderMaterialInstances` resource.
This patch also fixes a subtle issue that could cause mesh materials to
be incorrect if a `MeshMaterial3d<A>` was removed and replaced with a
`MeshMaterial3d<B>` material in the same frame. The problematic pattern
looks like:
1. `extract_mesh_materials<B>` runs and, seeing the
`Changed<MeshMaterial3d<B>>` condition, adds an entry mapping the mesh
to the new material to the untyped `RenderMeshMaterialIds`.
2. `extract_mesh_materials<A>` runs and, seeing that the entity is
present in `RemovedComponents<MeshMaterial3d<A>>`, removes the entry
from `RenderMeshMaterialIds`.
3. The material slot is now empty, and the mesh will show up as whatever
material happens to be in slot 0 in the material data slab.
This commit fixes the issue by splitting out `extract_mesh_materials`
into *three* phases: *extraction*, *early sweeping*, and *late
sweeping*, which run in that order:
1. The *extraction* system, which runs for each material, updates
`RenderMaterialInstances` records whenever `MeshMaterial3d` components
change, and updates a change tick so that the following system will know
not to remove it.
2. The *early sweeping* system, which runs for each material, processes
entities present in `RemovedComponents<MeshMaterial3d>` and removes each
such entity's record from `RenderMeshInstances` only if the extraction
system didn't update it this frame. This system runs after *all*
extraction systems have completed, fixing the race condition.
3. The *late sweeping* system, which runs only once regardless of the
number of materials in the scene, processes entities present in
`RemovedComponents<ViewVisibility>` and, as in the early sweeping phase,
removes each such entity's record from `RenderMeshInstances` only if the
extraction system didn't update it this frame. At the end, the late
sweeping system updates the change tick.
Because this pattern happens relatively frequently, I think this PR
should land for 0.16.
d55f47d00a