Add vsync to settings

[Germanunkol]

I get horrible stripes on the screen, sometimes.

I think enabling vsync will solve that problem... so we'll need to add it to the settings, I guess.

[michalove]

Isn't vsync enabled by default? I got stripes, too, when I ran the game in windowed mode. However, it was impossible to take screenshots of the stripes, so I guess that indeed had to do with vsync and the synchronization between graphics card and monitor.