update skpdiff visualization (image magnification with alpha mask)

tools/skpdiff/skpdiff_main.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #if SK_SUPPORT_OPENCL
 
 #define __NO_STD_VECTOR // Uses cl::vectpr instead of std::vectpr
@@ skipping 171 matching lines @@
     if (!FLAGS_csv.isEmpty()) {
         if (1 != FLAGS_csv.count()) {
             SkDebugf("csv flag expects one argument: <csv file>\n");
             return 1;
         }
     }
 
     SkDiffContext ctx;
     ctx.setDiffers(chosenDiffers);
 
+    ctx.setDifferenceDir(SkString("pdiff_alpha"));
+
     if (FLAGS_threads >= 0) {
         ctx.setThreadCount(FLAGS_threads);
     }
 
     // Perform a folder diff if one is requested
     if (!FLAGS_folders.isEmpty()) {
         ctx.diffDirectories(FLAGS_folders[0], FLAGS_folders[1]);
     }
 
     // Perform a pattern diff if one is requested
@@ skipping 13 matching lines @@
     }
 
     return 0;
 }
 
 #if !defined(SK_BUILD_FOR_IOS) && !defined(SK_BUILD_FOR_NACL)
 int main(int argc, char * argv[]) {
     return tool_main(argc, (char**) argv);
 }
 #endif