fix numerical overflows in 565 blends

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 56 matching lines @@
                               const SkPMColor* SK_RESTRICT src, int count,
                                U8CPU alpha, int /*x*/, int /*y*/) {
     SkASSERT(255 > alpha);
 
     if (count > 0) {
         do {
             SkPMColor sc = *src++;
             SkPMColorAssert(sc);
             if (sc) {
                 uint16_t dc = *dst;
-                unsigned dst_scale = 255 - SkMulDiv255Round(SkGetPackedA32(sc), alpha);
-                unsigned dr = SkMulS16(SkPacked32ToR16(sc), alpha) + SkMulS16(SkGetPackedR16(dc), dst_scale);
-                unsigned dg = SkMulS16(SkPacked32ToG16(sc), alpha) + SkMulS16(SkGetPackedG16(dc), dst_scale);
-                unsigned db = SkMulS16(SkPacked32ToB16(sc), alpha) + SkMulS16(SkGetPackedB16(dc), dst_scale);
-                *dst = SkPackRGB16(SkDiv255Round(dr), SkDiv255Round(dg), SkDiv255Round(db));
+                SkPMColor res = SkBlendARGB32(sc, SkPixel16ToPixel32(dc), alpha);
+                *dst = SkPixel32ToPixel16(res);
             }
             dst += 1;
         } while (--count != 0);
     }
 }
 
 /////////////////////////////////////////////////////////////////////////////
 
 static void S32_D565_Opaque_Dither(uint16_t* SK_RESTRICT dst,
                                      const SkPMColor* SK_RESTRICT src,
@@ skipping 144 matching lines @@
             proc = PlatformProcs565(flags);
             if (NULL == proc) {
                 proc = gDefault_565_Procs[flags];
             }
             break;
         default:
             break;
     }
     return proc;
 }