Basic mechanics for a secondary texcache, cleanup.

No need to set these values upon resuse.
Don't rehash a non-matching texture.
Use a secondary texture cache for hash failures.

Animation is pretty common in 2D games, keep it cached.
More aggressively decimate the second texcache.
Fix second texture cache key, duh.
Hash textures using cityhash rather than adding.

It should be more accurate, and for some games is faster.
For most it seems not hugely different.
Merge pull request #1505 from unknownbrackets/texcache

Add a "secondary" texture cache for 2d animation and clut changes