rebaseline_server: use just skpdiff, not Python Image Library

tools/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
 
 #include "SkBitmap.h"
 
 /**
  * Encapsulates an image difference metric algorithm that can be potentially run asynchronously.
  */
 class SkImageDiffer {
 public:
     SkImageDiffer();
     virtual ~SkImageDiffer();
 
     static const double RESULT_CORRECT;
     static const double RESULT_INCORRECT;
 
     struct Result {
         double result;
         int poiCount;
         SkBitmap poiAlphaMask; // optional
+        SkBitmap rgbDiffBitmap; // optional
+        SkBitmap whiteDiffBitmap; // optional
+        int maxRedDiff; // optional
+        int maxGreenDiff; // optional
+        int maxBlueDiff; // optional
         double timeElapsed; // optional
     };
 
     /**
      * Gets a unique and descriptive name of this differ
      * @return A statically allocated null terminated string that is the name of this differ
      */
     virtual const char* getName() const = 0;
 
     /**
      * Gets if this differ needs to be initialized with and OpenCL device and context.
      */
     virtual bool requiresOpenCL() const { return false; }
 
     /**
      * diff on a pair of bitmaps.
      * @param  baseline    The correct bitmap
      * @param  test        The bitmap whose difference is being tested
-     * @param  computeMask true if the differ is to attempt to create poiAlphaMask
+     * @param  computeAlphaMask true if the differ is to attempt to create poiAlphaMask
+     * @param  computeRgbDiff   true if the differ is to attempt to create rgbDiffBitmap
+     * @param  computeWhiteDiff true if the differ is to attempt to create whiteDiffBitmap
      * @return             true on success, and false in the case of failure
      */
-    virtual bool diff(SkBitmap* baseline, SkBitmap* test, bool computeMask,
-                      Result* result) const = 0;
+    virtual bool diff(SkBitmap* baseline, SkBitmap* test, bool computeAlphaMask,
+                      bool computeRgbDiff, bool computeWhiteDiff, Result* result) const = 0;
 };
 
 #endif