Add comments to clarify use of fInputColorIsOpaque

Bug: skia:7722
Change-Id: I9e2a70ac67947900897f6bb9237d6c3af1d03b7c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/413136
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed