Retina screen support

[bvssvni]

On retina screens, the frame buffer is twice the size of the internal window size in each direction. When one renders text, one might want to compensate for this. In piston-graphics this is done through the viewport library https://github.com/pistondevelopers/viewport

[bvssvni]

We might want to compute a column major transform matrix in the viewport library, since this is common for 3D.

[retep998]

Don't forget that Windows has arbitrary DPI scaling.

[bvssvni]

The viewport library assumes arbitrary draw width/height so it should be fine.