Add Gothic 2 using OpenGothic

[d10sfan]

This uses the OpenGothic project to add support for Gothic II. There were a few Linux-specific issues with the X11 renderer that I helped fix, and it seems to work pretty well now. The engine is still in-progress but so far it seems like basic game-play is very usable, them stating at least the first chapter is mostly playable.

So far, the only thing I've seen that are issues related directly to the engine are:

• Steam overlay does not work from Luxtorpeda, but launching the game from Steam works fine. It looks like steam has the overlay disabled for this game, so there might not be any way around that.

The engine package is at https://gitlab.com/d10sfan/luxtorpeda-opengothic

[d10sfan]

Mouse issues have been fixed, so other than the Steam Overlay issue, it's looking good.