Though the values are the same, this is cleaner, and more 'future' proof

This fixes a busy loop.

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.