Proper barrel sky distortion

Skies now use proper barrel distortion in both axes, rather than just the x-axis (i.e. just horizontally) as in all previous SRB2 versions. Credit goes to Nev3r for the original change and idea.

Was going to make an option for a non-barrel distorted sky, assuming barrel distortion is the default, but I just can't quite get it to work in a way that is pleasing to the eyes. So I've not bothered with it here for now, unless someone objects to barrel distorted skies for any reason?

See merge request !35