Replace GrExtractAlphaFragmentProcessor with DstIn compose processor

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 414 matching lines @@
     virtual Context* onCreateContext(const ContextRec&, void* storage) const;
 
     virtual bool onAsLuminanceColor(SkColor*) const {
         return false;
     }
 
     virtual bool onIsABitmap(SkBitmap*, SkMatrix*, TileMode[2]) const {
         return false;
     }
 
+    /** 
+     *  A helper for asFragmentProcessor() implementations. Many implementations want to compute
+     *  a color in a FP and then multiply that color by the incoming color's (usually paint color's)
+     *  alpha, ignoring the incoming r,g,b. This wraps a FP in a parent FP that does exactly that.
+     *  There will be no input color to the wrapped FP.
+     */
+    static const GrFragmentProcessor* MulFragmentProcesorOuputByInputAlpha(

[reed1, 2015/09/16 13:34:27]

Curious why this is on shader, and not on GrFragmentProcessor?

[bsalomon, 2015/09/21 15:29:50]

Done.

+        const GrFragmentProcessor*);
+
 private:
     // This is essentially const, but not officially so it can be modified in
     // constructors.
     SkMatrix fLocalMatrix;
 
     // So the SkLocalMatrixShader can whack fLocalMatrix in its SkReadBuffer constructor.
     friend class SkLocalMatrixShader;
     friend class SkBitmapProcShader;    // for computeTotalInverse()
 
     typedef SkFlattenable INHERITED;
 };
 
 #endif