SPF_REPLACE for particle functions

[Major Cooke]

Pull Request

SPF_REPLACE will respawn old particles anew when the particle limit is reached, overriding them. 'Nuff said.

I'm making it opt-in only so things like particle fountains don't eat away particles that otherwise would have their effect ruined by spontaneously poofing out of existence. Consider this a way of overriding lesser particles.

Also, rail shots come to mind... You could have a beautiful fireworks display and a single railgun shot that's long enough just eats them all up for supper. Not a fan of that.