Make pixel comparison utility in ReadPixelsTest shareable

Also make it support GrColorType

Change-Id: I2aecb82dd1b8e3bc942549f2023ff5cae9deb4f3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/227403
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
3 files changed