[cairo] Problem with unaligned clip regions

[Tristan Schmelcher]

To add some more information, the black area is created by
combine_src_u in pixman, and it is black because the corresponding
mask values are all zero. The mask comes from
_composite_unaligned_boxes in cairo-image-surface.c, which is called
from _composite_boxes, which is in turn called from
_clip_and_composite_boxes. That's about the limit that I can debug
this though since I don't really understand the box compositing code.