* gdk-pixbuf/pixops/pixops.c: (pixops_composite_nearest),
	(composite_pixel), (composite_line):
	* gdk-pixbuf/test-gdk-pixbuf.c: (simple_composite_test_one_type):
	Now that Owen explained the algorithm, I think I implemented it
	right for the cases where the destination pixbuf is not opaque.
	The test does seem to confirm it.