Add ability to extract YUV planes from SkImage

src/core/SkColorMatrixFilterRowMajor255.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 SkColorMatrixFilter_DEFINED
 #define SkColorMatrixFilter_DEFINED
 
 #include "SkColorFilter.h"
 
 class SK_API SkColorMatrixFilterRowMajor255 : public SkColorFilter {
 public:
     explicit SkColorMatrixFilterRowMajor255(const SkScalar array[20]);
 
+    /** Creates a color matrix filter that returns the same value in all four channels. */
+    static SkColorFilter* CreateSingleChannelOutput(const SkScalar row[5]);
+
     void filterSpan(const SkPMColor src[], int count, SkPMColor[]) const override;
     void filterSpan4f(const SkPM4f src[], int count, SkPM4f[]) const override;
     uint32_t getFlags() const override;
     bool asColorMatrix(SkScalar matrix[20]) const override;
     SkColorFilter* newComposed(const SkColorFilter*) const override;
 
 #if SK_SUPPORT_GPU
     const GrFragmentProcessor* asFragmentProcessor(GrContext*) const override;
 #endif
 
     SK_TO_STRING_OVERRIDE()
 
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkColorMatrixFilter)
 
 protected:
     void flatten(SkWriteBuffer&) const override;
 
 private:
+    SkColorMatrixFilterRowMajor255() {};
+
     SkScalar        fMatrix[20];
     float           fTranspose[20]; // for Sk4s
     uint32_t        fFlags;
 
-    void initState(const SkScalar array[20]);
+    void initState();
 
     typedef SkColorFilter INHERITED;
 };
 
 #endif