Simplify clipping rule
The new rule is:
- Output is clipped to the destination clip region.
- If a source image has the clip_sources property set, then there
is an additional step, after repeating and transforming, but before
compositing, where pixels that are not in the source clip are
rejected. Rejected means no compositing takes place (not that the
pixel is treated as 0). By default source clipping is turned off;
when they are turned on, only client-set clips are honored.
The old rules were unclear and inconsistently implemented.