The skybox rendering process has been replaced with portals instead. Those are generated after the first BSP tree pass by looking for existing sky visplanes at the time, and their windows are used to define new portals.
The skybox portals are still incomplete and cause visual glitches when masked elements are involved.
Split portal-related code to its own source files.
Most of the 2-line-specific setup has been moved to the function which adds a 2-line case. The portals should render as they used to so far, anyway.
Additionally, place some optimisations in both software and OpenGL; in particular one has been added for when all of back and front sector (floor and ceiling) is sky: since everything is "open" anyway, we can simply the usual checks involved.
Thankfully it was really just a copy+paste of the code I already tinkered with for the normal ceiling sky based thok barriers, but tweaked for floors instead
* Thok barriers with slopes now render fine in software
* The solid "sky walls" between different-height thok barriers adjacent to each other are gone. Forever.
Polyobject segs should ONLY be drawn if the polyobject itself is in the polylist of a subsector being rendered. That way you won't sometimes see polyobject walls through level boundaries, if you happen to be close enough to their pre-spawn locations outside the level (or in them, if you decided to go on a noclip journey).
Compiling errors fixed in this commit:
* Various cases of mixed declaration and statement code
* Implicit declaration of slope functions (read: you forgot to put "include "p_slopes.h" in MORE than a few places)
* an odd case of a bad fixed_t to float typecase, cause by using P_GetZAt directly inside FIXED_TO_FLOAT
* a few minor cases of bad unsigned-signed comparisons
* no prototypes for some of the new slope functions. For goodness sake Red, this is basic stuff!
To be specific: when a sector had a sloped ceiling and a colormap was
placed above it, the colormap wouldn't fill anything above where the
ceiling height is at the sector's midpoint. This is fixed.
