Render arrow on-the-fly instead of setting to a static file

[tjhorner]

Right now, the arrow overlay is literally just a PNG that OpenVR directly reads, so we can't really do any fancy modifications to the arrow (e.g. color, transparency, etc.). This is fine for now, but if we want to introduce more customization options in the future we should implement something that generates the arrow in code then sends it to OpenVR as an OpenGL texture.

Can use the work from this repo as a starting point for the OpenGL implementation stuff.