MKV : Export MPEG[12] sequence header present in CodecPrivate as extradata.

This fixes files where the muxer doesn't store sequence headers
as part of the stream.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/mkv/matroska_segment.cpp

@@ -796,6 +796,8 @@ bool matroska_segment_c::Select( mtime_t i_start_time )
                  !strcmp( p_tk->psz_codec, "V_MPEG2" ) )
         {
             p_tk->fmt.i_codec = VLC_CODEC_MPGV;
+            if( p_tk->i_extra_data )
+                fill_extra_data( p_tk );
         }
         else if( !strncmp( p_tk->psz_codec, "V_THEORA", 8 ) )
         {