It can easily happen that the lower sector has no lines below the checked area, in which case it would not set the dropoffz correctly. To prevent this, P_LineOpening must, when it checks the opening over a sector portal, actually calculate the dropoff to the lower portal itself by calling sector_t::NextLowestPointAt. It also means that FindRefPoint must calculate a proper reference point when one side of the line to be checked is part of a floor portal.
This fixes Clang complaining about unknown command option '-Wno-unused-but-set-variable' when compiling dumb.
Also I got no new warnings on Clang by excluding '-Wno-unused-result'.
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.
- added portal offsetting to all AproxDistance, AngleTo and Vec*To members of AActor.
- optimized displacement retrieval so that the most common case with no offset retrieves a constant null-vector which can be optimized away fully by the compiler.
- early out in P_GetOffsetPosition if there's no portal lines nearby, so that the common case can skip the traverser completely even on maps with line portals.
- made some minor changes to FPathTraverse so that the Add*Intercepts methods can be virtually overridden.
- removed the PortalTracer class because in its existing form it was too costly. Replaced with a P_GetOffsetPosition function that does the minimum required work to get to the translated destination and that's better suited for being called from the Vec*Offset methods. Other use cases will require some changes to FPathTraverse anyway, or some wrapping class like the FMultiBlock iterators.
- renamed sector_t::soundorg in centerspot, changed the type to a fixedvec2 and removed the CenterSpot #define.
Since this thing was used in lots of places that have nothing to do with sound the name made no sense. Having it as a fixed_t array also made it clumsy to use and the CenterSpot #define used a potentially dangerous type cast.
