Add 8888 fast SrcOver mode.

src/core/SkLinearBitmapPipeline_core.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkLinearBitmapPipeline_core_DEFINED
 #define SkLinearBitmapPipeline_core_DEFINED
 
@@ skipping 167 matching lines @@
 
     while (count >= 4) {
         stage->pointList4(xs, ys);
         xs = xs + fourDx;
         count -= 4;
     }
     if (count > 0) {
         stage->pointListFew(count, xs, ys);
     }
 }
+
+class sRGBFast {
+public:
+    static Sk4f VECTORCALL sRGBToLinear(Sk4f pixel) {
+        Sk4f l = pixel * pixel;
+        return Sk4f{l[0], l[1], l[2], pixel[3]};
+    }
+    static Sk4f VECTORCALL LinearTosRGB(Sk4f pixel) {
+        Sk4f s = pixel.sqrt();
+        return Sk4f{s[0], s[1], s[2], pixel[3]};
+    }
+};
+
 }  // namespace
 
 #endif // SkLinearBitmapPipeline_core_DEFINED