Make GrCaps::surfaceSupportsReadPixels return distinct values rather than flags.

Change-Id: I2744ca6cb0e9f1f2fd7d20e293400c788e7d4083
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/224960
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
8 files changed