cls.glconfig.isFullscreen was not updated when changing r_fullscreen
without a vid_restart. Starting in fullscreen and switching to
windowed mode would not release the mouse.
Mods calling trap_GetGlconfig() after a fullscreen toggle now get
the correct value for isFullscreen. (Note: Mods already got the
correct value at start up and after vid_restart.)
Reported by Mickaël "mickael9" Thomas.