Make cell corpse chunks use the real collision shapes of organelles

[hhyyrylainen]

as we'll soon have nice collision shapes for them would probably be nice to switch to using them for the cell corpse chunks as well.

For performance reasons it'd be nice to have an option in the game settings to revert back to using spheres (there can be quite many dropped chunk objects so it is plausible that someone would get better performance when they use spheres, and we might be able to increment the max chunks at once variable now with much more performance headroom we have).