Update SkLumaMaskXfermode partial coverage handling.

Use lerp-to-dst instead of straight coverage scaling to align the behavior with
the other transfer modes.

R=reed@google.com

Review URL: https://codereview.chromium.org/23453004

git-svn-id: http://skia.googlecode.com/svn/trunk@10927 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed