We were masking out the alpha bit in the key color not int
the source pixel, so colorkeying didn't work when the high byte
was != 0. For instance in the shutdown dialog in XP.
This includes:
* pixman region from SpiceRects
* rop2 enum
* solid fill
* solid fill with rop
* tiled fill
* tiled fill with rop
* blit
* blit with rop
* copy rect