decodebin: Always add a multiqueue in single-stream use-buffering pipelines

decodebin: Always add a multiqueue in single-stream use-buffering pipelines

If we are configured to use buffering and there is no demuxer in the chain, we
still want a multiqueue, otherwise we will ignore the use-buffering property.
In that case, we will insert a multiqueue after the parser or decoder - not
elsewhere, otherwise we won't have timestamps.

https://bugzilla.gnome.org/show_bug.cgi?id=764948