- 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.