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

src/core/SkBlitRow_D16.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBlitRow.h"
 #include "SkColorPriv.h"
 #include "SkDither.h"
@@ skipping 162 matching lines @@
     if (count > 0) {
         int src_scale = SkAlpha255To256(alpha);
         DITHER_565_SCAN(y);
         do {
             SkPMColor c = *src++;
             SkPMColorAssert(c);
             if (c)
             {
                 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);
                 int sg = SkGetPackedG32(c);
                 int sb = SkGetPackedB32(c);
                 sr = SkDITHER_R32To565(sr, dither);
                 sg = SkDITHER_G32To565(sg, dither);
                 sb = SkDITHER_B32To565(sb, dither);
 
                 int dr = (sr * src_scale + SkGetPackedR16(d) * dst_scale) >> 8;
@@ skipping 78 matching lines @@
     flags >>= 2;
 
     SkASSERT(flags < SK_ARRAY_COUNT(gDefault_565_ColorProcs));
 
     SkBlitRow::ColorProc16 proc = PlatformColorFactory565(flags);
     if (nullptr == proc) {
         proc = gDefault_565_ColorProcs[flags];
     }
     return proc;
 }