use some explicit types (bspbrush_t::ptr for pointer, bspbrush_t::container for container of pointers, and bspbrush_t::make_ptr to create one) so that we can swap it out with different pointers easier later
swap out bspbrush_t::ptr from unique_ptr to shared_ptr; it's not important yet, but it does not increase compilation time and will allow us to pass things to functions like SplitBrush without destroying the original as long as we still have a reference to it somewhere (important for ChopBrushes)
add get_positive_plane to mapface_t
instead of creating a whole copy of the bspbrush_t list, BrushBSP now just creates copies of the shared_ptrs which are then filtered down the tree; the originals are never modified by SplitBrushList and friends, only split into new brushes, so this is safe from what I can tell
portals now directly reference the mapface_ts
original_brushes now directly reference the mapbrush_ts
early exit in `BrushBSP` if brush list is empty (moved to top)
097b564cc6