gst/matroska/: Set DISCONT flag on first buffer after a discontinuity.

gst/matroska/: Set DISCONT flag on first buffer after a discontinuity.

Original commit message from CVS:
* gst/matroska/matroska-demux.c: (gst_matroska_demux_add_stream),
(gst_matroskademux_do_index_seek),
(gst_matroska_demux_handle_seek_event),
(gst_matroska_demux_parse_blockgroup_or_simpleblock):
* gst/matroska/matroska-ids.h:
Set DISCONT flag on first buffer after a discontinuity.
Fix newsegment events sent when seeking and honour KEY_UNIT
seek flag. Create pad with bogus caps if we don't recognise
the stream codec id.
* gst/matroska/matroska-demux.h:
Fix GObject macros.