XFA: Support PNG format in pdfium_test and add image diffing

samples/BUILD.gn

 # 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.
 
 group("samples") {
   deps = [
     ":pdfium_test",
   ]
 }
 
 executable("pdfium_test") {
   sources = [
+    "image_diff_png.cc",
     "pdfium_test.cc",
   ]
   deps = [
     "//third_party/pdfium",
   ]
 }
+
+executable("pdfium_diff") {
+  sources = [
+    "image_diff.cc",
+    "image_diff_png.cc",
+    "image_diff_png.h",
+  ]
+  deps = [
+    "//third_party/pdfium",
+  ]
+}