implement more blendmodes for skvm

Change-Id: Ib7286235249f9b4027c1c59c9fae268a9dffefcd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/278096
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Reed <reed@google.com>
4 files changed