Calling vout_Control() after vlc_object_find() or input_GetVouts is not
valid. It cannot be. There is no warranty that pf_control is valid (the
video output plugin could be initializing or deinitializing). Even if
there were a lock without the pf_control implementation (as with X11),
it still wouldn't work.

Anyway, in this particular case, we already have video-snapshot to do
the exact same thing in a safe manner.

There remain some issues with snapshots:
 - there is no synchronization of b_snapshoy with the video thread,
 - there is no/incomplete error handling,
 - there is no protection against multiple concurrent snapshot reqs.