Deal with unclosed sectors

[jmickle66666666]

Unfortunately there are a lot of unclosed sectors in Doom 2 at least. They need to still triangulate, even tho they are incomplete polygons. It would be trivial if not for the fact it makes tracing lines more difficult.

GZDoom manages to recreate them just like Chocolate Doom.

[jmickle66666666]

I am currently detecting lonely vertexes (only connected to one linedef), it's possible these always come in pairs, so I can matchmake them. Needs investigation