vout: inherit variables from configuration (fixes #7378)

vout: inherit variables from configuration (fixes #7378)

The VLC UIs use the VLC configuration instead of variables. That way,
updated values are propagated immediately, rather than when a video
output is created.

Since the snapshot-related variables have no callbacks, they only need
to be created if and when they are overriden/set. This only occurs with
the LibVLC media player snapshot function (which remains quite broken).