* set up linked sector portals so that everything that will eventually have to be considered is present, even though the software renderer currently can't handle those adequately.
* tag all skybox things with a type so that they can easily be distinguished at run time.
* fill in the linked portal types in xlat/eternity.txt.
Please note that these still require the portal to be set up in the map with Line_SetPortal. It will not create a new portal if none exists on any line with the given ID.
* linked portals may never have a z-offset so the parameter for that needs to be ignored.
* for interactive portals, handling z-displacements when some distance calculation reaches through a portal is way too extensive and problematic to ever have a chance of really working. If such a portal gets defined it will be changed to a teleport-only portal and a message printed.
- 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.
- did some cleanup on the portal interface on linedefs: All checks should go through isLinePortal (gameplay related) and isVisualPortal (renderer related) which then can decide on the actual data what to return.
- removed portal_passive because it won't survive the upcoming refactoring.
- removed all direct access to portal members of line_t.
- always use the precise (and fast) version of P_PointOnLineSide inside the renderer.
