Port SkBlitRow::Color32 to SkOpts.

src/opts/SkBlitRow_opts.h

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkBlitRow_opts_DEFINED
+#define SkBlitRow_opts_DEFINED
+
+#include "Sk4px.h"
+
+namespace SK_OPTS_NS {
+
+// Color32 uses the blend_256_round_alt algorithm from tests/BlendTest.cpp.
+// It's not quite perfect, but it's never wrong in the interesting edge cases,
+// and it's quite a bit faster than blend_perfect.
+//
+// blend_256_round_alt is our currently blessed algorithm.  Please use it or an analogous one.
+static void blit_row_color32(SkPMColor* dst, const SkPMColor* src, int count, SkPMColor color) {
+    unsigned invA = 255 - SkGetPackedA32(color);
+    invA += invA >> 7;
+    SkASSERT(invA < 256);  // We've should have already handled alpha == 0 externally.
+
+    Sk16h colorHighAndRound = Sk4px::DupPMColor(color).widenHi() + Sk16h(128);
+    Sk16b invA_16x(invA);
+
+    Sk4px::MapSrc(count, dst, src, [&](const Sk4px& src4) -> Sk4px {
+        return (src4 * invA_16x).addNarrowHi(colorHighAndRound);
+    });
+}
+
+}  // SK_OPTS_NS
+
+#endif//SkBlitRow_opts_DEFINED