[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>
6 files changed