Update SkLumaMaskXfermode partial coverage handling.

src/effects/SkLumaXfermode.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkLumaXfermode.h"
 #include "SkColorPriv.h"
 #include "SkFlattenableBuffers.h"
@@ skipping 58 matching lines @@
                                 int count, const SkAlpha aa[]) const {
     const SkPMColor* a = lumaOpA<SkPMColor>(fMode, src, dst);
     const SkPMColor* b = lumaOpB<SkPMColor>(fMode, src, dst);
 
     if (aa) {
         for (int i = 0; i < count; ++i) {
             unsigned cov = aa[i];
             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;
             }
         }
     } else {
         for (int i = 0; i < count; ++i) {
             dst[i] = luma_proc(a[i], b[i]);
         }
     }
 }
@@ skipping 122 matching lines @@
     // No background texture support.
     if (effect && !background) {
         *effect = GrLumaMaskEffect::Create(fMode);
         return true;
     }
 
     return false;
 }
 
 #endif // SK_SUPPORT_GPU