Fix the ordering of the systems introduced in #18734.

[pcwalton]

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 Unify RenderMaterialInstances and RenderMeshMaterialIds, and fix an associated race condition. #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.

[tychedelia]

Makes sense, apologies for missing the missing system on the last review. Thanks for tracking this down with all the churn.

[superdump]

I think it was me who made the chain comment. I didn’t realise that consequence indeed. Sorry for the lost time :/

[superdump]

error: public documentation for `ExtractMaterialsSet` links to private item `extract_mesh_materials`
   --> crates/bevy_pbr/src/material.rs:638:41
    |
638 | /// A [`SystemSet`] that contains all [`extract_mesh_materials`] systems.
    |                                         ^^^^^^^^^^^^^^^^^^^^^^ this item is private
    |
    = note: this link resolves only because you passed `--document-private-items`, but will break without
    = note: `-D rustdoc::private-intra-doc-links` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(rustdoc::private_intra_doc_links)]`

error: could not document `bevy_pbr`