revise row blits to keep intermediate precision so that color is preserved when blended against its…

src/core/SkBlitRow_D16.cpp

Index: src/core/SkBlitRow_D16.cpp
diff --git a/src/core/SkBlitRow_D16.cpp b/src/core/SkBlitRow_D16.cpp
index 9ac84c672df13f80c28b4855ad0afd1eac7d8f0e..648e0ea8bb16a439d4a51f99b969c589b69edd1a 100644
--- a/src/core/SkBlitRow_D16.cpp
+++ b/src/core/SkBlitRow_D16.cpp
@@ -180,7 +180,7 @@ static void S32A_D565_Blend_Dither(uint16_t* SK_RESTRICT dst,
             {
                 unsigned d = *dst;
                 int sa = SkGetPackedA32(c);
-                int dst_scale = SkAlpha255To256(255 - SkAlphaMul(sa, src_scale));
+                int dst_scale = SkAlphaMulInv256(sa, src_scale);
                 int dither = DITHER_VALUE(x);
 
                 int sr = SkGetPackedR32(c);