[graphite] Account for coverage when deciding to do in-shader blending

Change-Id: I8b24c0b67b85b5946c9dbee8ae78973ef154d5ac
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/747196
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: James Godfrey-Kittle <jamesgk@google.com>
2 files changed