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

include/core/SkShader.h

Index: include/core/SkShader.h
diff --git a/include/core/SkShader.h b/include/core/SkShader.h
index 4f3fd29693a8ce56096c528017b0416115abb7d0..92883f0afbdda7905c2e7837a0246ad2578c152d 100644
--- a/include/core/SkShader.h
+++ b/include/core/SkShader.h
@@ -386,6 +386,16 @@ public:
                              const SkMatrix* localMatrixOrNull, GrColor* paintColor,
                              GrEffect** effect) const;
 
+    /**
+     *  If the shader can represent its "average" luminance in a single color, return true and
+     *  if color is not NULL, return that color. If it cannot, return false and ignore the color
+     *  parameter.
+     *
+     *  Note: if this returns true, the returned color will always be opaque, as only the RGB
+     *  components are used to compute luminance.
+     */
+    bool asLuminanceColor(SkColor*) const;
+
 #ifdef SK_BUILD_FOR_ANDROID_FRAMEWORK
     /**
      *  If the shader is a custom shader which has data the caller might want, call this function
@@ -472,6 +482,9 @@ protected:
      */
     virtual Context* onCreateContext(const ContextRec&, void* storage) const;
 
+    virtual bool onAsLuminanceColor(SkColor*) const {
+        return false;
+    }
 private:
     // This is essentially const, but not officially so it can be modified in
     // constructors.