blitbuffer implementation with support for > 4bpp

A new blitbuffer implementation, featuring support for blitbuffers
with 4bit (as before), 8bit gray, 16bit gray, 16bit RGB, 24bit RGB,
32bit RGB(A) pixel values ("Color").

These buffers interoperate automatically. It is optimized to be sped
up by LuaJIT pretty well.

Rotation is now done using "virtual" blitbuffers. Same goes for
value inversion. These virtual blitbuffers are just overlays on other
blitbuffers which store the actual data.