exit_segnum is set during load_endlevel_data, which currently runs too
early for this to depend on the exit used, and thus to be player-unique.
That data should be loaded at need, when the level ends, rather than
during level setup. This can be addressed later, when support for
multiple exits is improved.
Replace calls to window_close(Game_wind) with returning window_event_result::close to game handler. Applies to when DoEndGame() is called, DoGameOver() is called, aborting in the kmatrix screen (multiplayer game) during AdvanceLevel() and playing one demo frame causes playback to stop in GameProcessFrame(). Closing a window within its handler is problematic - it can result in an unstable state.