Add GrCaps::getTestingCombinations

This will decrease our reliance on GrPixelConfig for testing.

Bug: skia:6718
Change-Id: I951f57cce229792d994391dbba74dd206bbfb923
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/230893
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
12 files changed