extend SkShader to report a luminance-color to be used for gamma correction

src/effects/gradients/SkGradientShaderPriv.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkGradientShaderPriv_DEFINED
 #define SkGradientShaderPriv_DEFINED
 
@@ skipping 219 matching lines @@
     uint8_t     fGradFlags;
 
     struct Rec {
         SkFixed     fPos;   // 0...1
         uint32_t    fScale; // (1 << 24) / range
     };
     Rec*        fRecs;
 
     void commonAsAGradient(GradientInfo*, bool flipGrad = false) const;
 
+    virtual bool onAsLuminanceColor(SkColor*) const SK_OVERRIDE;
+
     /*
      * Takes in pointers to gradient color and Rec info as colorSrc and recSrc respectively.
      * Count is the number of colors in the gradient
      * It will then flip all the color and rec information and return in their respective Dst
      * pointers. It is assumed that space has already been allocated for the Dst pointers.
      * The rec src and dst are only assumed to be valid if count > 2
      */
     static void FlipGradientColors(SkColor* colorDst, Rec* recDst,
                                    SkColor* colorSrc, Rec* recSrc,
                                    int count);
@@ skipping 214 matching lines @@
     GrGLProgramDataManager::UniformHandle fColorStartUni;
     GrGLProgramDataManager::UniformHandle fColorMidUni;
     GrGLProgramDataManager::UniformHandle fColorEndUni;
 
     typedef GrGLEffect INHERITED;
 };
 
 #endif
 
 #endif