video_output/vout_subpictures.c: Fix filters appearing and disappearing when loaded for the first time.

The sys->filter_chain_update and sys->source_chain_update interacted with each other, because
the code was not carefull enough on when it is safe to overload 'sub-source' with the value of
'sub-filter'. The 'sub-filter' chain can also be a 'sub-source', but not the otherway around.