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

src/core/SkSpriteBlitter_RGB16.cpp

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #include "SkSpriteBlitter.h"
 #include "SkBlitRow.h"
@@ skipping 12 matching lines @@
                                                unsigned src_scale) {
     uint16_t dc = *dst;
     unsigned sa = SkGetPackedA32(sc);
     unsigned dr, dg, db;
 
     if (255 == sa) {
         dr = SkAlphaBlend(SkPacked32ToR16(sc), SkGetPackedR16(dc), src_scale);
         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;
     }
     *dst = SkPackRGB16(dr, dg, db);
 }
 
 #define D16_S32A_Blend_Pixel(dst, sc, src_scale) \
     do { if (sc) D16_S32A_Blend_Pixel_helper(dst, sc, src_scale); } while (0)
 
@@ skipping 316 matching lines @@
                 } else {
                     blitter = allocator->createT<Sprite_D16_SIndex8A_Blend>(source, alpha);
                 }
             }
             break;
         default:
             break;
     }
     return blitter;
 }