Unify stream and demux
Sometimes it is desirable to prioritize an access over an access_demux, or a demux over a stream_filter.
access_t (access) and stream_t (stream_filter) have been unified already. demux_t should be unified with them as well to achieve that.
Concretely, this would imply that:
- on the one hand, access and access_demux capabilities are merged, and
- on the other hand, stream_filter and demux capabilities are merged too.
(Also this would make the directory demuxer totally redundant.)
One problem is handling of ES output, required for access_demux and demux, but not always available. (For instance, it is not available with vlc_stream_NewURL().)