Use jspngopt and pako to create reproducible PNG files for Chrome as well

The combination of jspngopt and pako should eliminate possible causes for
different PNG encodings, although the core reason for #325 remains unknown.
Pako has poorer compression rates than native libz, but optimization can
counter that effect, and actually reduce the size of the screenshots.

The screenshots for LimitControls and UnsupportedCmds on Firefox used to
exhibit subpixel rendering before, for reasons unknown.  The regenerated
versions don't exhibit this.  See #324 for a discussion.

dockers/Screenshotter/screenshotter.js

@@ -3,8 +3,10 @@
 var childProcess = require("child_process");
 var fs = require("fs");
 var http = require("http");
-var path = require("path");
+var jspngopt = require("jspngopt");
 var net = require("net");
+var pako = require("pako");
+var path = require("path");
 var selenium = require("selenium-webdriver");
 
 var app = require("../../server");
@@ -271,10 +273,15 @@ function takeScreenshot(key) {
         }
         var file = path.join(dstDir, key + "-" + opts.browser + ".png");
         var deferred = new selenium.promise.Deferred();
-        fs.writeFile(file, img.buf, function(err) {
+        var opt = new jspngopt.Optimizer({
+            pako: pako
+        });
+        var buf = opt.bufferSync(img.buf);
+        fs.writeFile(file, buf, function(err) {
             if (err) {
                 deferred.reject(err);
-            } else {
+            }
+            else {
                 deferred.fulfill();
             }
         });

dockers/Screenshotter/screenshotter.sh

@@ -8,7 +8,7 @@
 # directly.
 
 status=0
-for browserTag in firefox:2.46.0; do
+for browserTag in firefox:2.46.0 chrome:2.46.0; do
     browser=${browserTag%:*}
     image=selenium/standalone-${browserTag}
     echo "Starting container for ${image}"

package.json

@@ -20,7 +20,9 @@
     "jasmine-node": "2.0.0-beta4",
     "js-yaml": "^3.3.1",
     "jshint": "^2.5.6",
+    "jspngopt": "^0.1.0",
     "less": "~1.7.5",
+    "pako": "0.2.7",
     "nomnom": "^1.8.1",
     "selenium-webdriver": "^2.46.1",
     "uglify-js": "~2.4.15"