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

GrokImageCompression/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,chamois94/gst-plugins-good,vatavuserban/gst-plugins-good,GrokImageCompression/gst-plugins-good,sebras/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,StreamUtils/gst-plugins-good,sebras/gst-plugins-good,ikonst/gst-plugins-good,kittee/gst-plugins-good,GStreamer/gst-plugins-good,froggatt/gst-plugins-good-m,hizukiayaka/gst-plugins-good,StreamUtils/gst-plugins-good,froggatt/gst-plugins-good-m,hizukiayaka/gst-plugins-good,Kurento/gst-plugins-good,shelsonjava/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,sebras/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,shelsonjava/gst-plugins-good,rawoul/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,chamois94/gst-plugins-good,shelsonjava/gst-plugins-good,rawoul/gst-plugins-good,kittee/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,kittee/gst-plugins-good,shelsonjava/gst-plugins-good,chamois94/gst-plugins-good,kittee/gst-plugins-good,Kurento/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,Kurento/gst-plugins-good,pexip/gst-plugins-good,chamois94/gst-plugins-good,ikonst/gst-plugins-good,rawoul/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,vatavuserban/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,vatavuserban/gst-plugins-good,stfl/gst-plugins-good,sebras/gst-plugins-good,rawoul/gst-plugins-good,froggatt/gst-plugins-good-m,ikonst/gst-plugins-good,ikonst/gst-plugins-good,Kurento/gst-plugins-good,pexip/gst-plugins-good,stfl/gst-plugins-good,froggatt/gst-plugins-good-m,vatavuserban/gst-plugins-good,Kurento/gst-plugins-good,stfl/gst-plugins-good