This includes:
* allow one sided portal linedefs to be crossable when part of a polyobject. Due to the limitations, two-sided linedefs won't work here. For general use this is still not allowed because making them passable would require some crippling fudging.
* delay portal finalization until after polyobjects have been spawned.
* the camera interpolation also needs to handle angle differences.
The code is still not 100% complete - the most important thing that is still missing is proper handling of P_CheckPosition through arbitrary portals.
- store portal data in a separate structure.
- store portal data in savegames because some of this will be changeable later.
- run a cleanup pass after all portals have been created to weed out broken ones.
- add a definition type that's compatible with Eternity Engine's line portal types.
- swapped arg[2] and arg[3] of Line_SetPortal, because the type is more significant than the alignment.
