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

src/core/SkSpriteBlitter_RGB16.cpp

Index: src/core/SkSpriteBlitter_RGB16.cpp
diff --git a/src/core/SkSpriteBlitter_RGB16.cpp b/src/core/SkSpriteBlitter_RGB16.cpp
index d0d3001cbb925f76563020b50e05306b70e7cf00..ebc905f5b9dbd4785d68f187d8c95b73a17c6b80 100644
--- a/src/core/SkSpriteBlitter_RGB16.cpp
+++ b/src/core/SkSpriteBlitter_RGB16.cpp
@@ -30,7 +30,7 @@ static inline void D16_S32A_Blend_Pixel_helper(uint16_t* dst, SkPMColor sc,
         dg = SkAlphaBlend(SkPacked32ToG16(sc), SkGetPackedG16(dc), src_scale);
         db = SkAlphaBlend(SkPacked32ToB16(sc), SkGetPackedB16(dc), src_scale);
     } else {
-        unsigned dst_scale = 255 - SkAlphaMul(sa, src_scale);
+        unsigned dst_scale = SkAlphaMulInv256(sa, src_scale);
         dr = (SkPacked32ToR16(sc) * src_scale + SkGetPackedR16(dc) * dst_scale) >> 8;
         dg = (SkPacked32ToG16(sc) * src_scale + SkGetPackedG16(dc) * dst_scale) >> 8;
         db = (SkPacked32ToB16(sc) * src_scale + SkGetPackedB16(dc) * dst_scale) >> 8;