Add brush bounds fast test for axial planes.
Move collision model types from quake2world.h into cm_types.h, and include cm_types.h in shared.h. This reduces the size of quake2world.h, and lets the collision model code be more cohesive.
Rename user_cmd_t to pm_cmd_t, as that's what it is.
Rename network serialization functions for pm_cmd_t.