A very simple utility, that switches any X11 window to fullscreen mode without grabbing mouse/keyboard and allowing window switching.
Tested with Sauerbraten and several other SDL games.
Just focus on the desired window and run fullscreenize :)
It will do the trick.
Also you can run fullscreenize with the --execute switch.
In this case, fullscreenize will run the application for you.
$ fullscreenize --execute sauerbraten
If failed, try setting windowed mode in your game`s settings and try again.