gst/qtdemux/qtdemux.*: Queue up new segment events instead of sending them from the...

gst/qtdemux/qtdemux.*: Queue up new segment events instead of sending them from the seeking thread.

Original commit message from CVS:
* gst/qtdemux/qtdemux.h (struct _GstQTDemux):
* gst/qtdemux/qtdemux.c (gst_qtdemux_do_seek): Queue up new
segment events instead of sending them from the seeking thread.
Fixes #559288.
(gst_qtdemux_push_pending_newsegment): New helper, sends out
queued newsegment events.
(gst_qtdemux_loop_state_movie): Voilà, call it here. Only need to
call it here, as we only seek when looping, and only push in the
movie state.