Add 8888 fast SrcOver mode.

src/core/SkLinearBitmapPipeline_core.h

Index: src/core/SkLinearBitmapPipeline_core.h
diff --git a/src/core/SkLinearBitmapPipeline_core.h b/src/core/SkLinearBitmapPipeline_core.h
index 2759f0bb8ccc728472bb68a23d72c8bedf073007..0ced1a70b9815160d82ba2014260382f80c0deca 100644
--- a/src/core/SkLinearBitmapPipeline_core.h
+++ b/src/core/SkLinearBitmapPipeline_core.h
@@ -185,6 +185,19 @@ void span_fallback(Span span, Stage* stage) {
         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