Update SkLumaMaskXfermode partial coverage handling.

src/effects/SkLumaXfermode.cpp

Index: src/effects/SkLumaXfermode.cpp
diff --git a/src/effects/SkLumaXfermode.cpp b/src/effects/SkLumaXfermode.cpp
index df0a663c3b224f80934455dc176efd94b9409e23..88b0ddfbc341c3621df6f2e1836260dcc7616caa 100644
--- a/src/effects/SkLumaXfermode.cpp
+++ b/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;
             }