- pix-dither-2bpp-specprocedure
- in - PPIX - 8bpp/graymap
- in - LOWCLIP - lower clipping point, should be 0 or a small number.
- in - UPCLIP - upper clipping point, should be 255 or slightly less.
- in - CMAP? - generate colormap, 0 or 1.
- returns: dithered ppix or #f if error.