There used to be a bug (discussed in #40), that ALT was still set after
using ALT-Tab. Thus when next pressing enter fullscreen was toggled.
This should now be fixed by unsetting the modifiers when focus is
regained (SDL_ACTIVEEVENT or SDL_WINDOWEVENT_FOCUS_GAINED).
New CVar "in_kbd" to set the layout for the keyboard. SDL 1.2
doesn't offer any way to determine it, and we need this feature
to use the same key for toggling the console independent of the
keyboard layout.
The old "in_nograb" from the Linux backend is still supported.