udpate dox for colorfilter fragment processor requirements

include/core/SkColorFilter.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 SkColorFilter_DEFINED
 #define SkColorFilter_DEFINED
 
@@ skipping 126 matching lines @@
      */
     static SkColorFilter* CreateComposeFilter(SkColorFilter* outer, SkColorFilter* inner);
 
     /**
      *  A subclass may implement this factory function to work with the GPU backend.
      *  If it returns true, then 1 or more fragment processors will have been appended to the
      *  array, each of which has been ref'd, so that the caller is responsible for calling unref()
      *  on them when they are finished. If more than one processor is appended, they will be
      *  applied in FIFO order.
      *
+     *  The fragment processor(s) must each return their color as a premul normalized value
+     *  e.g. each component between [0..1] and each color component <= alpha.
+     *
      *  If the subclass returns false, then it should not modify the array at all.
      */
     virtual bool asFragmentProcessors(GrContext*, SkTDArray<GrFragmentProcessor*>*) const {
         return false;
     }
 
     SK_TO_STRING_PUREVIRT()
 
     SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP()
     SK_DEFINE_FLATTENABLE_TYPE(SkColorFilter)
 
 protected:
     SkColorFilter() {}
 
 private:
     typedef SkFlattenable INHERITED;
 };
 
 #endif