DVB sync

The problem

When watching live DVB, data is delivered strictly at the broadcasters
speed. We cannot change it through server commands. Our local systems clock
usually runs slightly faster or slower than that, causing a/v fifos to run
empty or full after a few minutes.

Standard network buffering control only handles the first case by pausing
the engine (not nice). The latter case ends up in severe stuttering and an
a/v lag of several seconds (annoying).

I tried quite a few differnt algorithms, and this one made it:
a 3 point controller.

There is a target buffer fill window with a center and some tolerated width:

Minimum:
  definition: 1 second
  safety: clamped to 38% of fio size
  action: switch playback speed to 99.5%

Center:
  definition: 2 seconds
  safety: clamped to 73% fifo size
  action: switch to normal playback speed

Maximum:
  definition: 3 seconds
  safety: clamped to 98% fifo fill
  action: switch playback speed to 100.5%

If the usual dvb audio to video muxing delay is more than 1 second, center
time is increased. On low bitrate radio, window width is increased.

Real TVs do adjust playback audio sampling rate to follow delivery speed.
Some PC sound cards can do that, too. It could be implemented transparently
(although I don't know yet how). This comes quite close, resampling audio to
stretch.

That half percent is large enough to cover clock deviation, and it is small
enough not to cause audible pitch bending. Speed control consists of
adjusting SCR and telling audio out. Doing just the first will cause
metronom to drop and/or insert whole audio frames, not nice with music.

BTW. this one needs demux_ts to send BUF_FLAG_FRAME_START.

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