CSG Make Room operation

CSG Make Room

With the help of Garux (thanks to him!), the CSG “Make Room” operation was added.

It makes possible to create a room from a brush by surrounding it, which is very handy to create skyboxes. See the related merge request thread for details about implementation.

The feature was also ported to DarkRadiant to help our friends from The Dark Mod and the Doom 3 mapping community.