Add support for HiDPI displays

By scaling the canvas width/height by window.devicePixelRatio (or 1 for a fallback) we can enable HiDPI support.

Optionally, we can also scale the line width on line 13 in ShaderOverlayView.coffee, but IMO it isn't needed.

Before & After example:

![Shadershop before and after HiDPI support](https://cloud.githubusercontent.com/assets/1525809/5267464/1b978370-7a49-11e4-9b20-669cbf51f8bb.png)