We can get rid of shadow blitbuffers. Screen.bb is now fb.bb.
All bpp conversion is now done by the blitbuffer abstraction layer.
Rotation mechanism is adapted to this.
This should finish the work to make all globals to local variables.
That allows LuaJIT to properly compile things by interning the
references to the relevant parts (rather than looking up globals
all the time which stops a trace).
