Add a workaround for PowerVRRogue to never disable color writes

Instead we leave color writes enabled and use a blend state that
preserves the dst color. This allows us to re-enable msaa ccpr on
PowerVR.

Bug: skia:
Change-Id: I1e902d695ad483ffb13dff6a7920749e307b49c7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/229387
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
11 files changed