Make skpaint->grpaint flow work for composing draws

include/core/SkShader.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkShader_DEFINED
 #define SkShader_DEFINED
 
@@ skipping 297 matching lines @@
 
     /**
      *  Returns a GrFragmentProcessor that implements the shader for the GPU backend. NULL is
      *  returned if there is no GPU implementation.
      *
      *  The GPU device does not call SkShader::createContext(), instead we pass the view matrix,
      *  local matrix, and filter quality directly.
      *
      *  The GrContext may be used by the to create textures that are required by the returned
      *  processor.
+     *
+     *  The returned GrFragmentProcessor should expect an unpremultiplied input color and
+     *  produce a premultiplied output.
      */
     virtual const GrFragmentProcessor* asFragmentProcessor(GrContext*,
                                                            const SkMatrix& viewMatrix,
                                                            const SkMatrix* localMatrix,
                                                            SkFilterQuality,
                                                            GrProcessorDataManager*) 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
@@ skipping 110 matching lines @@
     SkMatrix fLocalMatrix;
 
     // So the SkLocalMatrixShader can whack fLocalMatrix in its SkReadBuffer constructor.
     friend class SkLocalMatrixShader;
     friend class SkBitmapProcShader;    // for computeTotalInverse()
 
     typedef SkFlattenable INHERITED;
 };
 
 #endif