[graphite] Add BlendModeBlenderBlock::AddBlock

This CL also converts the two uses for the missing child blenders to be AddKnownModeBlend calls. There doesn't seem to be any reason to keep those as fully fledge sk_blend calls.

Bug: b/251493570
Change-Id: I77d50e4a5af65ada642405f3cce51e1415e05521
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/762977
Reviewed-by: Nicolette Prevost <nicolettep@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
4 files changed