Fix detection of when we will have mixed sampled coverage

Change-Id: I32de8bdea7b7aafd202f44a2036872f25d531ae3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/253763
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
1 file changed