Fixed Particle Rain Collision

WARNING : THIS WILL INCREASE PERFORMANCE REQUIREMENTS (ALSO, MAPS MUST BE SET TO USE THIS RAIN TYPE!) By default, GPU accelerated Rain has no Collision in NMRIH. After some digging into how it works within L4D2, where this is featured most prominently, I could not manage to duplicate the effects. They use a much more advanced set of rules and modifiers. SO! I decided to try my best and rig up what I could. In theory, there may be an even more optimised method, but it's bugged by default and needs to be patched. This mod allows you to use GPU particle rain, and snow, and other similar effects without issue in func_precipitation. It looks pretty good, and runs pretty well (at least, for me. only 1-2ms more overhead.)