Now SSA subtitle pause is working even with karaoke.

Otherwise we'll collapse and expand the same directory over and over.

It will avoid ABI breakage for aesthetics reasons.

This provides a pause with a maximal latency of one call
to pf_demux from the demuxer.

This allows a more reactive pause (independant of *-caching value). It is
not yet instantaneous as vout/aout still play their buffer.

- This is needed for proper seek/pause/fast forward/...
- The decoder/packetizer do not need to scale packet length anymore as the
decoder thread do it.

No functionnal changes yet.

There is no functionnal changes + cosmetics.