Make blurrect_compare GM work on GPUs without renderable A8

Change-Id: I8965a6a417bc7b228e723d8e53a5ff46f8c7badf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/240836
Auto-Submit: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
1 file changed