[graphite] Use Compose helper to implement dithering

Bug: 251493570
Change-Id: Ieb32c60a1a8fcbfb6060a44cf13821ef5006498d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/756997
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
7 files changed