Rework transfer pixels tests to be more robust.

Make them query caps for supported read/write info and do CPU
conversions before uploading/after reading.

Removes use of GrColor so in theory could be used to test
non-8888 color types.

Bug: skia:6718

Change-Id: Icf9d0b778348a4e960fbfec49e1308b21e45a051
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/227497
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
2 files changed