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

src/core/SkBlitter_ARGB32.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"
 #include "SkShader.h"
@@ skipping 219 matching lines @@
         return;
     }
 
     uint32_t* device = fDevice.writable_addr32(x, y);
     uint32_t  color = fPMColor;
 
     if (alpha != 255) {
         color = SkAlphaMulQ(color, SkAlpha255To256(alpha));
     }
 
+#ifdef SK_SUPPORT_LEGACY_BROKEN_LERP
     unsigned dst_scale = 255 - SkGetPackedA32(color);
+#else
+    unsigned dst_scale = SkAlpha255To256(255 - SkGetPackedA32(color));
+#endif
     size_t rowBytes = fDevice.rowBytes();
     while (--height >= 0) {
         device[0] = color + SkAlphaMulQ(device[0], dst_scale);
         device = (uint32_t*)((char*)device + rowBytes);
     }
 }
 
 void SkARGB32_Blitter::blitRect(int x, int y, int width, int height) {
     SkASSERT(x >= 0 && y >= 0 && x + width <= fDevice.width() && y + height <= fDevice.height());
 
@@ skipping 435 matching lines @@
             SkBlitRow::Proc32 proc = (255 == alpha) ? fProc32 : fProc32Blend;
             do {
                 shaderContext->shadeSpan(x, y, span, 1);
                 proc(device, span, 1, alpha);
                 y += 1;
                 device = (uint32_t*)((char*)device + deviceRB);
             } while (--height > 0);
         }
     }
 }