fix numerical overflows in 565 blends

src/core/SkBlitRow_D16.cpp

Index: src/core/SkBlitRow_D16.cpp
diff --git a/src/core/SkBlitRow_D16.cpp b/src/core/SkBlitRow_D16.cpp
index 1b2be06d12590f2c419739a9eaf5b8a3b94d2a47..e052b3500a2477e0369e0ebb252f8df1112d085b 100644
--- a/src/core/SkBlitRow_D16.cpp
+++ b/src/core/SkBlitRow_D16.cpp
@@ -74,11 +74,8 @@ static void S32A_D565_Blend(uint16_t* SK_RESTRICT dst,
             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);