Merge clip rects in GrReducedClip

Merges the intersection of all rect clips into a single clip rect
element. This is also the first step toward better handling of the
subtractive elements.

Bug: skia:
Change-Id: I36bd9c256874917adb68f43e8faddc609e65f37a
Reviewed-on: https://skia-review.googlesource.com/64380
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
4 files changed