Unroll color inversion

This cleans up all the inversion and re-inversion. Grayscale blitbuffers are 0=black, 255=white now. If the underlying framebuffer has it the other way around (like on KDX, K2, K3), only then we invert when blitting onto the framebuffer's blitbuffer. In all other cases - no inversion.

This has consequences for the specification of color values. Those are now expected to be, well, color values. Corresponing PR for koreader is following shortly.