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

src/effects/SkLumaXfermode.cpp

@@ -76,7 +76,8 @@ void SkLumaMaskXfermode::xfer32(SkPMColor dst[], const SkPMColor src[],
             if (cov) {
                 unsigned resC = luma_proc(a[i], b[i]);
                 if (cov < 255) {
-                    resC = SkAlphaMulQ(resC, SkAlpha255To256(cov));
+                    resC = SkFastFourByteInterp256(resC, dst[i],
+                                                   SkAlpha255To256(cov));
                 }
                 dst[i] = resC;
             }