Finish implementing blendmodes for skvm

Lifted directly from SkRasterPipeline_opts.h, transcribing into skvm.

Change-Id: I777b5d86f11e571b8d7faff38a6e5b0e3c9cc6a3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/278376
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
4 files changed