There's still a race resulting in blank materials whenever a material of
type A is added on the same frame that a material of type B is removed.
PR #18734 improved the situation, but ultimately didn't fix the race
because of two issues:
1. The `late_sweep_material_instances` system was never scheduled. This
PR fixes the problem by scheduling that system.
2. `early_sweep_material_instances` needs to be called after *every*
material type has been extracted, not just when the material of *that*
type has been extracted. The `chain()` added during the review process
in PR #18734 broke this logic. This PR reverts that and fixes the
ordering by introducing a new `SystemSet` that contains all material
extraction systems.
I also took the opportunity to switch a manual reference to
`AssetId::<StandardMaterial>::invalid()` to the new
`DUMMY_MESH_MATERIAL` constant for clarity.
Because this is a bug that can affect any application that switches
material types in a single frame, I think this should be uplifted to
Bevy 0.16.
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