Fix issue with some keys not working correctly when remapped with custom keybindings

The new input system doesn't play nice with the existing key rebinding screen (need to talk to @CIB about how that should be handled), making it currently unsuitable for the main game loop. This workaround prevents the new system handling keyboard input in the main loop,instead only handling mouse input.

Fixes http://smf.cataclysmdda.com/index.php?topic=4145.0