updating devicePixelScaling() method to take optional number arg

Author: Lauren McCarthy

lib/p5.js

@@ -1,4 +1,4 @@
-/*! p5.js v0.4.2 March 20, 2015 */
+/*! p5.js v0.4.2 March 23, 2015 */
 (function (root, factory) {
   if (typeof define === 'function' && define.amd)
     define('p5', [], function () { return (root.returnExportsGlobal = factory());});
@@ -2244,7 +2244,11 @@ amdclean['environment'] = function (require, core, constants) {
   };
   p5.prototype.devicePixelScaling = function (val) {
     if (val) {
-      this._pixelDensity = window.devicePixelRatio || 1;
+      if (typeof val === 'number') {
+        this._pixelDensity = val;
+      } else {
+        this._pixelDensity = window.devicePixelRatio || 1;
+      }
     } else {
       this._pixelDensity = 1;
     }