add ImageShader, sharing code with its Bitmap cousin

src/image/SkImageShader.h

Index: src/image/SkImageShader.h
diff --git a/src/image/SkImageShader.h b/src/image/SkImageShader.h
new file mode 100644
index 0000000000000000000000000000000000000000..fdd7976aad5aa2cf207fe5c7ce56402b93bd6cd6
--- /dev/null
+++ b/src/image/SkImageShader.h
@@ -0,0 +1,44 @@
+/*
+ * 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 SkImageShader_DEFINED
+#define SkImageShader_DEFINED
+
+#include "SkImage.h"
+#include "SkShader.h"
+
+class SkImageShader : public SkShader {
+public:
+    static SkShader* Create(const SkImage*, TileMode tx, TileMode ty, const SkMatrix* localMatrix);
+
+    bool isOpaque() const override;
+    size_t contextSize() const override;
+
+    SK_TO_STRING_OVERRIDE()
+    SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkImageShader)
+
+#if SK_SUPPORT_GPU
+    const GrFragmentProcessor* asFragmentProcessor(GrContext*, const SkMatrix& viewM,
+                                                   const SkMatrix*, SkFilterQuality,
+                                                   GrProcessorDataManager*) const override;
+#endif
+
+protected:
+    void flatten(SkWriteBuffer&) const override;
+    Context* onCreateContext(const ContextRec&, void* storage) const override;
+
+    SkAutoTUnref<const SkImage> fImage;
+    const TileMode              fTileModeX;
+    const TileMode              fTileModeY;
+
+private:
+    SkImageShader(const SkImage*, TileMode tx, TileMode ty, const SkMatrix* localMatrix);
+
+    typedef SkShader INHERITED;
+};
+
+#endif