It's right there in the game's graphical options, is it not?
Try changing it in the settings.txt file (or equivalent) in whatever the mac has as the equivalent to the documents folder where your savegames are also stored.
Played with the settings in there, thanks. You can create some glorious crashes with that file ;-D
It appears it's the multisampling setting. Changing it to 0 indeed fixes the problem. Thanks all for the help!