Make RegionOp a non-legacy GrMeshDrawOp

Also add a test factory function.

Change-Id: I612973312824a4ba358965ce8378160a980b86fa
Reviewed-on: https://skia-review.googlesource.com/22213
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
4 files changed