iOS: No fullscreen

[georgefst]

Unfortunately I couldn't get fullscreen to work with any WebKit browsers. Even document.documentElement.webkitRequestFullscreen on iPad, which is the one thing that apparently should work, as of version 12. Currently pressing Monpad's fullscreen button locks up the page completely, requiring a refresh. We disabled it in 7637f76, which worked at the time, but something has changed here, because my iPad is now showing the fullscreen button again, even though it still crashes when the button is pressed.