tests/examples/seek/seek.c: Fix various seeking bugs (Slider was not updating when doing a non flushing seek, Reverse...

Original commit message from CVS:
2007-02-27  Julien MOUTTE  <julien@moutte.net>

* tests/examples/seek/seek.c: (do_seek), (set_update_scale),
(msg_segment_done): Fix various seeking bugs (Slider was not
updating when doing a non flushing seek, Reverse playback
on segment seek was wrong).