Add shader-based GaussianEdgeShader for reveal case

include/effects/SkRRectsGaussianEdgeShader.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 SkRRectsGaussianEdgeShader_DEFINED
+#define SkRRectsGaussianEdgeShader_DEFINED
+
+#include "SkShader.h"
+
+class SkRRect;
+
+class SK_API SkRRectsGaussianEdgeShader {
+public:
+    /** Returns a shader that applies a Gaussian blur depending on distance to the edge
+     *  of the intersection of two round rects.
+     *  Currently this is only useable with round rects that have the same radii at
+     *  all the corners and for which the x & y radii are equal.
+     *  Raster will draw nothing.
+     *  
+     *  The coverage geometry that should be drawn should be no larger than the intersection
+     *  of the bounding boxes of the two round rects. Ambitious users can omit the center
+     *  area of the coverage geometry if it is known to be occluded.
+     */
+    static sk_sp<SkShader> Make(const SkRRect& first,
+                                const SkRRect& second,
+                                SkScalar radius, SkScalar pad);
+
+    SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP()
+
+private:
+    SkRRectsGaussianEdgeShader(); // can't be instantiated
+};
+
+#endif