fast path for pathops

Add faster path for simple but common path ops:
- intersect two rects
- all ops where one operand is empty

R=halcanary@google.com

Bug: skia:8049
Change-Id: I2a516d095feae8478ee9433262c9c77e5e18ce81
Reviewed-on: https://skia-review.googlesource.com/132929
Auto-Submit: Cary Clark <caryclark@skia.org>
Reviewed-by: Cary Clark <caryclark@skia.org>
Commit-Queue: Cary Clark <caryclark@skia.org>
3 files changed