Skybox hotfix

This branch fixes how, for a skybox with both a centerpoint and viewpoint, the first person view code did not take the centerpoint's angle into account. So when the centerpoint's angle is NOT 0 (or a multiple of 360), this results in the skybox "moving the wrong way" when you're viewing the skybox this way and moving about.

...yeah sorry guys, I commited to a copy of next again, whoops. Cherry-pick the relevant commit into master if this works out fine - after all, this shouldn't affect netgames I think (it's purely about rendering after all)