hlsdemux/others: Bad drift compensation for live streams

Submitted by Sebastian Dröge @slomo

Link to original bug (#759333)

Description

Currently with live streams we will just fall behind more and more based on clock drift and also because of rebuffering due to BUFFERING messages.

The only time when we resync (in hls to the 3rd newest fragment, no idea for the others) is when we fall off the Manifest completely. If the Manifest is huge (I have one here that has about 1 hour of fragments), we would get up to one hour behind.

Instead we should probably start resyncing earlier. Like once we are 3 or 6 or $number fragments behind the one we would resync to, or 1 minute or whatever worth of fragments behind the fragment that we would resync to.

It doesn't seem a good idea to have live streams go that far away from being live.

I'll provide a patch for HLS later, but unfortunately the same work has to be redone for DASH and the others individually.

DASH fortunately has an optional extension that allows to compensate for this based on the server clock, but if that is not available we should also do something like I proposed above.