[henu]

I'm making a game on rbfx, which uses Diligent. The game world is practically endless and streamed in chunks, and I noticed an annoying hitch every time the player moves from one chunk to another.

The hitch comes from releasing the old chunks: a few hundred terrain patches at once, each with its own vertex buffer. For every released buffer, VAOCache::ClearStaleKeys() scans every key of each PSO the buffer was drawn with, to find the stale ones. All the terrain patches share the same few PSOs, so each of those lists holds thousands of keys, and the scans add up.

This PR keeps the keys in m_PSOToKey and m_BuffToKey in sets instead of vectors, so ClearStaleKeys() erases just the stale keys, without scanning the others.

I have tested this in my game, and the hitches when the player crosses a chunk border are now gone. I tested this both visually and by measuring the times. Releasing the old chunks' terrain patches took 78 ms on average, now 4 ms. The measuring was done in rbfx's copy of Diligent.

I first made this PR to rbfx, but was asked to make it here instead. This PR is the same change ported onto this repository.