[graphite] Implement AddBlendModeColorFilter with the Blend helper

Bug: 251493570
Change-Id: Ib73da65e12862f9c3b5e599484f3449321dae3b0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/756057
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
3 files changed