qtdemux: Update segment.start after key-unit seek

qtdemux: Update segment.start after key-unit seek

When doing key uint seek, qtdemux calls gst_qtdemux_adjust_seek
to get proper offset. And then this offset is set to
segment.position and segment.time in gst_qtdemux_perform_seek but
segment.start is not updated.

After that, application sends segment query,
qtdemux sets start and stop to query using gst_segment_to_stream_time. Due
to the wrong value in segment.start, the stop position is smaller than
it should.

https://bugzilla.gnome.org/show_bug.cgi?id=746822