Fixed sky-sky thok barriers showing HOM (turns out the floor and ceiling share...

Fixed sky-sky thok barriers showing HOM (turns out the floor and ceiling share the same plane, so I've modified the code to account for this)

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.