Add GrDisableColorXPFactory::makeXferProcessor()

This allows us to access the actual XP for working directly with
pipelines.

Bug: skia:
Change-Id: I71e7c2e9d69f464685e28b4f4a85847f0496b864
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/208345
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
2 files changed