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

This reverts commit d3c9245454ed998b997a215c57b4f70efd2b569d.

Reason for revert: the world is on fire!

Original change's description:
> 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>

TBR=ethannicholas@google.com,reed@google.com

Change-Id: Ia75857bb81a500784a3d92dbfa2bc59292763868
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206390
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
14 files changed