Blend functions as SkSL built ins.

GPU-only for right now.

Use in GrGLSLBlend::AppendMode.

Remove unused GrGLSLBlend::AppendRegionOp.

Change-Id: I23affe255a1adafa55f3a07ca6291dc84e18e213
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/254678
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
8 files changed