pixman_composite_trapezoids() is supposed to composite across the
entire destination, but it actually only composites across the extent
of the trapezoids. For operators such as ADD or OVER this doesn't
matter since a zero source has no effect on the destination. But for
operators such as SRC or IN, it does matter.
So for such operators where a zero source has an effect, don't clip to
the trap extents.
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