This commit makes Bevy use change detection to only update
`RenderMaterialInstances` and `RenderMeshMaterialIds` when meshes have
been added, changed, or removed. `extract_mesh_materials`, the system
that extracts these, now follows the pattern that
`extract_meshes_for_gpu_building` established.
This improves frame time of `many_cubes` from 3.9ms to approximately
3.1ms, which slightly surpasses the performance of Bevy 0.14.
(Resubmitted from #16878 to clean up history.)
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Co-authored-by: Charlotte McElwain <charlotte.c.mcelwain@gmail.com>
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
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