bootanimation: performance/speedup enhancements

-Don't cache textures if they're expected to use a lot of VRAM (rmcc)
-allow using RGB565 instead of ARGB8888:
RGB565 uses considerably less memory, and together with texture cache,
it allows us to offer a really smooth bootanimation on not so powerful
devices.

Change-Id: I397eaf425d4ba0eeda954de6795d57395be8e4e7