Backport PNG output format to origin/master branch.

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',
-      '<(DEPTH)/v8/tools/gyp/v8.gyp:v8_libplatform',
+    'defines' : [
+      'PNG_PREFIX',
+      'PNGPREFIX_H',
+      'PNG_USE_READ_MACROS',
     ],
     'include_dirs': [
       '<(DEPTH)',
       '<(DEPTH)/v8',
       '<(DEPTH)/v8/include',
     ],
   },
   'targets': [
     {
       'target_name': 'pdfium_test',
+      'type': 'executable',
+      'dependencies': [
+        'fx_lpng',
+        '../pdfium.gyp:pdfium',
+        '<(DEPTH)/v8/tools/gyp/v8.gyp:v8_libplatform',
+      ],
       'sources': [
         'pdfium_test.cc',
+        'image_diff_png.cc',
+      ],
+    },
+    {
+      'target_name': 'pdfium_diff',
+      'type': 'executable',
+      'variables': { 'enable_wexit_time_destructors': 1, },
+      'dependencies': [
+        'fx_lpng',
+        '../pdfium.gyp:pdfium',
+        '../third_party/third_party.gyp:pdfium_base',
+      ],
+      'include_dirs': [
+        '../../',
+      ],
+      'sources': [
+        'image_diff.cc',
+        'image_diff_png.h',
+        'image_diff_png.cc',
+      ],
+    },
+    {
+      'target_name': 'fx_lpng',
+      'type': 'static_library',
+      'dependencies': [
+        '../pdfium.gyp:fxcodec',
+      ],
+      'include_dirs': [
+        '../core/src/fxcodec/fx_zlib/include/',
+      ],
+      'sources': [
+        'fx_lpng/include/fx_png.h',
+        'fx_lpng/src/fx_png.c',
+        'fx_lpng/src/fx_pngerror.c',
+        'fx_lpng/src/fx_pngget.c',
+        'fx_lpng/src/fx_pngmem.c',
+        'fx_lpng/src/fx_pngpread.c',
+        'fx_lpng/src/fx_pngread.c',
+        'fx_lpng/src/fx_pngrio.c',
+        'fx_lpng/src/fx_pngrtran.c',
+        'fx_lpng/src/fx_pngrutil.c',
+        'fx_lpng/src/fx_pngset.c',
+        'fx_lpng/src/fx_pngtrans.c',
+        'fx_lpng/src/fx_pngwio.c',
+        'fx_lpng/src/fx_pngwrite.c',
+        'fx_lpng/src/fx_pngwtran.c',
+        'fx_lpng/src/fx_pngwutil.c',
       ],
     },
   ],
 }