Currently, the specialized pipeline cache maps a (view entity, mesh
entity) tuple to the retained pipeline for that entity. This causes two
problems:
1. Using the view entity is incorrect, because the view entity isn't
stable from frame to frame.
2. Switching the view entity to a `RetainedViewEntity`, which is
necessary for correctness, significantly regresses performance of
`specialize_material_meshes` and `specialize_shadows` because of the
loss of the fast `EntityHash`.
This patch fixes both problems by switching to a *two-level* hash table.
The outer level of the table maps each `RetainedViewEntity` to an inner
table, which maps each `MainEntity` to its pipeline ID and change tick.
Because we loop over views first and, within that loop, loop over
entities visible from that view, we hoist the slow lookup of the view
entity out of the inner entity loop.
Additionally, this patch fixes a bug whereby pipeline IDs were leaked
when removing the view. We still have a problem with leaking pipeline
IDs for deleted entities, but that won't be fixed until the specialized
pipeline cache is retained.
This patch improves performance of the [Caldera benchmark] from 7.8×
faster than 0.14 to 9.0× faster than 0.14, when applied on top of the
global binding arrays PR, #17898.
[Caldera benchmark]: https://github.com/DGriffin91/bevy_caldera_scene
5e569af2d0