Update Porter Duff analysis to match LCD logic.

Bug: skia:
Change-Id: Ib222c37f7ebb307752806829742a3b6278dbae3d
Reviewed-on: https://skia-review.googlesource.com/17410
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
1 file changed