Add methods to SkBlenderBase to identify SkBlendMode-based blends.

Change-Id: Ia0ef90dd62b15f679791be6b9d14ed24a59ea8af
Bug: skia:12080
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/420821
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Mike Reed <reed@google.com>
2 files changed