add glBSP + glVis source and go binding

[tinogoehlert]

In order to eliminate the external glbsp and glvis dependency, we sahould include the source, add cgo bindings and interface it in a way where we can work with buffers instead of files.

glVis is needed for the rendering optimization and we don't want to rely on the presence of the GL_PVS lump (because it's mostly not there).