Optimize alpha-only CLUTs, common for fonts.

This optimizes the case where fonts are drawn using animation (common in
RPGs, for example), and textures have to be decoded over and over.

Could probably be faster with SIMD.
Improve GPU invalidation interface, check fonts.

When a font is rendered, tell the GPU about it, but don't rehash.

This not only improves perf (less hashing) but makes font changes more
immediate (no deferred hashing.)  But only if they use sceFont.