Allows PCR<=>PTS<=>offset estimation/calculation
Right now the calculation is very naive, but can be extended later on
without disrupting the code in tsdemux/mpegtsbase
* Don't take into account packets that arrived at the same time as
previous ones for clock skew estimation
* Add convenience method for processing the next ts packet
buffer timestamps are converted to GstClockTime to cover pcr/pts wraps.
multiple pcr/pts wraps are handled with an index which ensures at most
a single pcr wraparound between two entries.
the last seen pcr is recorded to have a nearby index point for short seeks
resuming playback might be delayed if the postion is not a keyframe
TODO: replace manual packet scanning and parsing in the initial duration estimation