Reland "[graphite] Add caps to support readPixels and writePixels."

This is a reland of commit 54076422cc483dd7d5b406af64c1ee9f65d4609c

Original change's description:
> [graphite] Add caps to support readPixels and writePixels.
>
> Adds the checks for base support, and the colortype needed.
>
> Bug: b/238756873
> Change-Id: I77f29a610a5411eba2017d3aaf59c62a3a05b53a
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/589116
> Commit-Queue: Jim Van Verth <jvanverth@google.com>
> Reviewed-by: Greg Daniel <egdaniel@google.com>

Bug: b/238756873
Change-Id: I8706ca656aa751b5cdec5c486f29d056f830be15
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/589404
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
6 files changed