Make skpaint->grpaint flow work for composing draws

include/gpu/GrFragmentProcessor.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrFragmentProcessor_DEFINED
 #define GrFragmentProcessor_DEFINED
 
@@ skipping 13 matching lines @@
 class GrFragmentProcessor : public GrProcessor {
 public:
     /**
     *  In many instances (e.g. SkShader::asFragmentProcessor() implementations) it is desirable to
     *  only consider the input color's alpha. However, there is a competing desire to have reusable
     *  GrFragmentProcessor subclasses that can be used in other scenarios where the entire input
     *  color is considered. This function exists to filter the input color and pass it to a FP. It
     *  does so by returning a parent FP that multiplies the passed in FPs output by the parent's
     *  input alpha. The passed in FP will not receive an input color.
     */
-    static const GrFragmentProcessor* MulOuputByInputAlpha(const GrFragmentProcessor*);
+    static const GrFragmentProcessor* MulOutputByInputAlpha(const GrFragmentProcessor*);
+
+    /**
+     *  Similar to the above but it modulates the output r,g,b of the child processor by the input
+     *  rgb and then multiplies all the components by the input alpha. This effectively modulates
+     *  the child processor's premul color by a unpremul'ed input and produces a premul output
+     */
+    static const GrFragmentProcessor* MulOutputByInputUnpremulColor(const GrFragmentProcessor*);
+
+    /**
+     * Returns a parent fragment processor that adopts the passed fragment processor as a child. The
+     * parent will ignore its input color and instead feed the passed in color as input to the
+     * child.
+     */
+    static const GrFragmentProcessor* OverrideInput(const GrFragmentProcessor*, GrColor);
 
     GrFragmentProcessor()
         : INHERITED()
         , fUsesLocalCoords(false)
         , fNumTexturesExclChildren(0)
         , fNumTransformsExclChildren(0) {}
 
     ~GrFragmentProcessor() override;
 
     GrGLFragmentProcessor* createGLInstance() const;
@@ skipping 146 matching lines @@
     int                                          fNumTransformsExclChildren;
 
     // TODO: These must convert their processors to pending-execution refs when the parent is
     // converted (do this automatically in GrProgramElement?).
     SkTArray<const GrFragmentProcessor*, true>   fChildProcessors;
 
     typedef GrProcessor INHERITED;
 };
 
 #endif