This just adds the API. Additional work will be required to actually
create an SkShaderSnippet for the runtime effect.
The SkBlenderID will be used in the combination system to specify the
Blender runtime effect as a combination candidate.
Bug: skia:12701
Change-Id: I524a2fa7a00ece6d9422891bcbc82ce0237679e3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/540170
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
cb3009126d