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.
