GPU support for SkMixers

Bug: skia:
Change-Id: Ic92f0b3c4613b3d599d59924d93cd4da8d161e89
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/204445
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
14 files changed