Add some sort of text rendering system

[jvanverth]

Some sort of simple bitmap (or distance field) glyph rendering system would be helpful for displaying debug text in the window.

[jvanverth]

ocornut's imgui might be a good placeholder until we get our own system working.

[jvanverth]

Can use bmfont (http://www.angelcode.com/products/bmfont/) and convert to distance field (http://bitsquid.blogspot.com/2010/04/distance-field-based-rendering-of.html)