* Add support for clip in/out rects on GPU.
* Remove CPU polygon clipping code.
* Fix all known vertex cracks and pixel sparkling.

There is still some code cleanup to do, and missing features, such as:
 * Nested complex clip regions.
 * Switching angle gradients to rectangles.
 * Complex clip regions + border radius.

These *should* be fairly simple to implement on top of these changes.