Replace GrExtractAlphaFragmentProcessor with DstIn compose processor

include/core/SkShader.h

Index: include/core/SkShader.h
diff --git a/include/core/SkShader.h b/include/core/SkShader.h
index a95e7e713e4dedb79c8ea865e217c070f200bca8..ec598f3f6a938c0b5c44f98e4a185c5bc642663f 100644
--- a/include/core/SkShader.h
+++ b/include/core/SkShader.h
@@ -432,6 +432,15 @@ protected:
         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.