Check that coverage FPs are compatible with alpha as coverage

Change-Id: Ic3b6a02248d571e82f6729827c199d358fb75114
Reviewed-on: https://skia-review.googlesource.com/8508
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
7 files changed