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

src/core/SkBlitter_A8.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 "SkCoreBlitters.h"
 #include "SkColorPriv.h"
@@ skipping 258 matching lines @@
             }
         }
     }
 }
 
 static inline uint8_t aa_blend8(SkPMColor src, U8CPU da, int aa) {
     SkASSERT((unsigned)aa <= 255);
 
     int src_scale = SkAlpha255To256(aa);
     int sa = SkGetPackedA32(src);
-    int dst_scale = 256 - SkAlphaMul(sa, src_scale);
+    int dst_scale = SkAlphaMulInv256(sa, src_scale);
 
     return SkToU8((sa * src_scale + da * dst_scale) >> 8);
 }
 
 void SkA8_Shader_Blitter::blitAntiH(int x, int y, const SkAlpha antialias[],
                                     const int16_t runs[]) {
     SkShader::Context* shaderContext = fShaderContext;
     SkXfermode*        mode = fXfermode;
     uint8_t*           aaExpand = fAAExpand;
     SkPMColor*         span = fBuffer;
@@ skipping 134 matching lines @@
     while (--height >= 0) {
         memcpy(dst, src, width);
         dst += dstRB;
         src += srcRB;
     }
 }
 
 const SkPixmap* SkA8_Coverage_Blitter::justAnOpaqueColor(uint32_t*) {
     return nullptr;
 }