Particle rendering

Building off #11, a styler for textured quads will be created. It will take lots of individual vertices and then expand them using a geometry shader. For the time being, this styler will use PBR shading and simple `destination * (1 - alpha) + source * alpha` blending (which requires that the particles be depth-sorted). #8 + #7 might change this in the future. There should also be a way to switch between ordinary and billboarded quads (separate view matrices for pos vs tan/bitan?).