Revert "Revert "Reland "GPU support for SkMixers"""

This reverts commit a6dd1ebd42cf647b34e002ddf4f676b96ceda708.

Bug: skia:
Change-Id: I382dd9cf189a69e844f656c80be4e269f8d6005d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206388
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
14 files changed