Simplify clipping rule

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.