make OpenCL optional for skpdiff

experimental/skpdiff/SkImageDiffer.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkImageDiffer_DEFINED
 #define SkImageDiffer_DEFINED
 
@@ skipping 14 matching lines @@
      */
     virtual const char* getName() = 0;
 
     /**
      * Gets if this differ is in a usable state
      * @return True if this differ can be used, false otherwise
      */
     bool isGood() { return fIsGood; }
 
     /**
+     * Gets if this differ needs to be initialized with and OpenCL device and context.
+     */
+    virtual bool requiresOpenCL() { return false; }
+
+    /**
      * Wraps a call to queueDiff by loading the given filenames into SkBitmaps
      * @param  baseline The file path of the baseline image
      * @param  test     The file path of the test image
      * @return          The results of queueDiff with the loaded bitmaps
      */
     int queueDiffOfFile(const char baseline[], const char test[]);
 
     /**
      * Queues a diff on a pair of bitmaps to be done at some future time.
      * @param  baseline The correct bitmap
@@ skipping 38 matching lines @@
     virtual SkIPoint* getPointsOfInterest(int id) = 0;
 
 
 
 protected:
     bool fIsGood;
 };
 
 
 #endif