Clip regions

[vancluever]

One addition that I want now that we have gradients coming up (very soon!) are clip regions.

Basically this is modeled after the same concept in Cairo (or Skia), and allows drawing outside of a particular shape, or with a general paint function.

Currently, to get the same functionality in z2d, you'd need to do multiple drawing passes on the surface, possibly using multiple intermediary surfaces and mask surfaces (made easier now using the lower-level compositor, but still).