Previously, line-based portals accessed global
ceilingclip/floorclip before the current seg was rendered.
This means clipping for such portals would be based on
whichever seg happened to render before. Additionally,
ceilingclip/floorclip have more to do with clipping the
visplanes than the height of the wall between (for
instance, using these values doesn't work for two-sided
lines that create a "window" between the planes). The
correct approach that should always work is using the
midtexture's height from inside of R_RenderSegLoop.
All typedefs for structs that were present in other header
files have been moved to here. (Except node_t because the
renderer and netcode both define node_t LOL.)
Vissprites are now only clipped against their respective portal's geometry obtained from their BSP run.
Additionally, if a portal is provided, they're clipped to the portal's clip boundaries.
The work on this branch should conclude after a pair of remaining glitches are fixed.
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.
