- 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.
