Plumb method for ejecting brass; add ejecting shotgun casings at correct time

[Thanson16]

Break out ejecting brass into a dedicated method. Trigger this method after every shot. Add in appropriate delay so that shotgun casings are ejected during the pump action rather than when the shot is fired.

I was a little uncertain what to do around the ComputeAmbience and SetSoundEnvironment stuff - if my EjectedBrass method needs those parts, let me know.

Tested on Linux, both as a player and in spectator mode.

Closes #917 , related to #909