XFA: Support PNG format in pdfium_test and add image diffing

samples/samples.gyp

 # Copyright 2014 PDFium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'target_defaults': {
     'type': 'executable',
     'dependencies': [
       '../pdfium.gyp:pdfium',
     ],
     'include_dirs': ['<(DEPTH)'],
   },
   'targets': [
     {
       'target_name': 'pdfium_test',
       'sources': [
         'pdfium_test.cc',
+        'image_diff_png.cc',
+      ],
+    },
+    {
+      'target_name': 'pdfium_diff',
+      'type': 'executable',
+      'variables': { 'enable_wexit_time_destructors': 1, },
+      'dependencies': [
+        '../pdfium.gyp:fxcodec',
+        '../third_party/third_party.gyp:safemath',
+      ],
+      'include_dirs': [
+        '../../',
+      ],
+      'sources': [
+        'image_diff.cc',
+        'image_diff_png.h',
+        'image_diff_png.cc',
       ],
     },
   ],
 }