Fix the pixman renderer with respect to output transforms

Fix the pixman renderer with respect to output transforms

Commit fa1b3055 introducted a regression in the pixman renderer.  In
particular, it would not draw properly with any output transform other
than normal, 180, 180-flipped, or 270-flipped.  This patch fixes this
regression.  The weston-scalar program appears to work normally at all
rotations with this patch.  Therefore, this patch should fix the regression
while maintaining the added functionality from fa1b3055.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>