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.

Low DPI:
![lowdpi](https://f.cloud.github.com/assets/327081/821734/786f1b16-efe0-11e2-8092-97dfaa648fe6.png)

High DPI:
![hidpi](https://f.cloud.github.com/assets/327081/821736/7ccd99da-efe0-11e2-971a-3e4124874e8c.png)