adding new GM to demostrate new shadows

src/core/SkShadowShader.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 SkShadowShader_DEFINED
+#define SkShadowShader_DEFINED
+
+class SkLights;
+class SkShader;
+
+class SK_API SkShadowShader {
+public:

[robertphillips, 2016/07/29 16:30:52]

Fix comment. 

// This shader combines the diffuse color in 'diffuseShader' with the shadows
determined by the 'povDepthShader' and the shadow maps stored in each of the
lights in 'lights'

?

There isn't really a shape here. This is just a shader that you're applying to a
rect.

[vjiaoblack, 2016/07/29 17:39:31]

Done.

+    /** Returns a shader that lights the shape, colored by the diffuseShader,
+     *  and generates shadows with the set of lights provided.
+     */
+    static sk_sp<SkShader> Make(sk_sp<SkShader> povDepthShader,
+                                sk_sp<SkShader> diffuseShader,
+                                sk_sp<SkLights> lights);
+
+    SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP()
+};
+
+#endif