s&box scene system - progress 4 (hackweek)

For Facepunch's Hackweek I played around with a particle system for the Scene System in s&box. I was going to do terrain but Matt had already beaten me to it. I like making particle systems, so I've avoided doing it before because it seemed self-indulgent when we already had a particle system. I'm pretty happy with how it turned out. I think it's already good enough for 95% of things people would want to do with particles, while being a bit more accessible. The particle updates are fully multithreaded, and fed to the renderer as point vertices, where the geometry shader creates the camera facing sprite triangles.