Enable High DPI display support.

These changes make the webGL context render at the native resolution
for high DPI displays like on the retina MacBook Pro and Chromebook
Pixel.

Babylon/babylon.engine.js

@@ -27,7 +27,7 @@
         this._workingContext = this._workingCanvas.getContext("2d");
 
         // Viewport
-        this._hardwareScalingLevel = 1.0;
+        this._hardwareScalingLevel = 1.0 / window.devicePixelRatio || 1.0;
         this.resize();
 
         // Caps