gst/matroska/: Fix Dirac mapping.  I had previously added a VfW-type mapping, but it looks like Dirac will get a nati...

Original commit message from CVS:
* gst/matroska/matroska-demux.c:
* gst/matroska/matroska-ids.h:
* gst/matroska/matroska-mux.c:
Fix Dirac mapping.  I had previously added a VfW-type
mapping, but it looks like Dirac will get a native Matroska
mapping, and this is the most likely method.