3D GUI backend(s)

[nic-hartley]

Not sure exactly what these will look like yet, but probably:

• Generate a chunk of a UV sphere in a vertex shader, radius large enough for the distortion to be visible but not frustratingly so
• Render onto it in scanlines, from fixed font data, in a fragment shader
• Postprocess some gentle bloom

The goal here is to make it look like a CRT, while keeping it crisp and readable. Also, the filters will be optional and ideally individually disableable.