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 R_CreateColormap to R_CreateLightTable
* Replace extra_colormaps array with next/prev pointer chain
* Remove foundcolormaps; instead store lumpnum in extracolormap_t
* Add properties to extracolormap_t for portability