gst/matroska/matroska-demux.*: Close the current segment if we're doing a non-flushing seek and send the close-segmen...

Original commit message from CVS:
* gst/matroska/matroska-demux.c: (gst_matroska_demux_reset),
(gst_matroska_demux_element_send_event),
(gst_matroska_demux_handle_seek_event), (gst_matroska_demux_loop):
* gst/matroska/matroska-demux.h:
Close the current segment if we're doing a non-flushing seek and send
the close-segment and the new segment of the seek from the streaming
thread.