Since the hull depth needs to be set for the hull to be useful, it makes
sense to move the code into the same place that allocates new hulls (to me,
anyway).
I got rather tired of there being multiple definitions of mostly compatible
plane types (and I need a common type anyway). dplane_t still exists for
now because I want to be careful when messing with the actual bsp format.
generice, and also more in line with the rest of quake's clipping system
(ie, support the entity size selected hulls). I'm not /entirly happy with
what's here, but it's a start, and the force fields in customtf work /much/
better :)
