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

krichter722/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,krichter722/vlc,shyamalschandra/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,krichter722/vlc,shyamalschandra/vlc,shyamalschandra/vlc,krichter722/vlc,jomanmuk/vlc-2.2,xkfz007/vlc,xkfz007/vlc,shyamalschandra/vlc,shyamalschandra/vlc,shyamalschandra/vlc,krichter722/vlc,xkfz007/vlc,shyamalschandra/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc,xkfz007/vlc,xkfz007/vlc,jomanmuk/vlc-2.2,xkfz007/vlc,krichter722/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,xkfz007/vlc,vlc-mirror/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,krichter722/vlc