If we end up with a segment with an internal time that varies from the supposed
one, this could be for two reasons:
* We guess-timated the wrong segment to go to when advancing or switching
variants. In that case we try to find the actual segment to go to (just before
this change).
* There was a complete playlist change (for whatever reason) and we can't find a
replacement. In that case we want to carry on playback from this position but
need to remember that we moved (by setting the stream to DISCONT, and
resetting the new mapping).
Fixes playback on several broken stream
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6961>