tsdemux: recalculate new segment after byte seek

tsdemux: recalculate new segment after byte seek

The seeking start time is approximated from the seek offset in bytes
using the accumulated PCR observations, so on a VBR stream there might
be a big difference between the actual PCR and the estimated one after
the seek. This might result in a long wait to skip all out of segments
packets.

Instead we just recalculate the new segment to start at the first PTS
after the seek, so that playback starts immediatly.