fullscreen works: the mindbending solution

This is not as responsive as I'd like it to be. That is caused by the FS event
being enqueued to be processed in vo_check_events, which then calls command.c
who will call vo_control(_, VOCTRL_FULLSCREEN, 0) which in turn has to wait
again for the GUI thread.

The lag is noticeable but bearable. Will look for a better solution.