[graphite] Add blendMode uniform to blend/compose shader

Bug: skia:12701
Change-Id: I648948875a2e9401c0c73cdf4c9c8053c9ba69af
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/510227
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
6 files changed