Fix CompatibleCoverageAsAlpha flag when reducing src-over to src.

Change-Id: I087ff64e0f23aee15ac2bf7b9d3c450e28400cef
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/274036
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
4 files changed