gst/matroska/: Implement Dirac muxing into Matroska comforming to the spec, i.e. put all Dirac packages up to a pictu...

Original commit message from CVS:
* gst/matroska/matroska-ids.h:
* gst/matroska/matroska-mux.c: (gst_matroska_pad_free),
(gst_matroska_mux_handle_dirac_packet),
(gst_matroska_mux_write_data):
Implement Dirac muxing into Matroska comforming to the spec, i.e.
put all Dirac packages up to a picture into a Matroska block.
TODO: Implement writing of the ReferenceBlock Matroska elements,
currently the Dirac muxing is only 100% correct if Matroska version 2
is selected for muxing.