[graphite] Make circular_rrect_clip snippet output coverage instead.

Allows it to be combined with any clip shader.

Bug: b/238762307
Change-Id: Ia5aff9d84716961e84405cdee4bf9bf8aac45ba9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/894417
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
4 files changed