Some old 2D games change palettes very frequently, which causes the
texture to be updated for each render.
This change uses a hash of the palette to detect changes, and include
the palette type in the cache key to distinguish textures of different
depth.
This consolidates some of the work done for TARGET_NO_NVMEM and
feat/no-direct-memmap. If nvmem is disabled at compile time or alloc
fails _nvmem_enabled() will return false. Various other fixes
and cleanups all around.
Flyinghead