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.

![hidpi](https://f.cloud.github.com/assets/327081/281945/1776a01e-9184-11e2-8157-e9b208096bdd.png)
![lowdpi](https://f.cloud.github.com/assets/327081/281946/177dc9fc-9184-11e2-90bd-1124adbf06e3.png)