This has the advantage of getting entity_t out of the particle system,
and much easier to read math. Also, it served as a nice test for my
particle physics shaders (implemented the ideas in C). There's a lot of
code that needs merging down: all but the actual drawing can be merged.
There's some weirdness with color ramps, but I'll look into that later.
r_local.h, r_shared.h, client.h and render.h at the same time (couldn't get
away from that:/) verly likely to be lots of breakage (eg, player and eye
models are NOT checksummed atm), but everything builds
