Make Mock GrBackendFormat use GrColorType (instead of GrPixelConfig)

The switch to GrColorType does mean that we can no longer represent compressed backend formats in the Mock backend surfaces.

This will require a Chrome CL before it can land in Skia.

TBR=bsalomon@google.com
Change-Id: Ie4e2d4826f960664a21d3de79933eb1cb5d06896
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/225538
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
13 files changed