matroskaenc: write DisplayUnit element to better match the spec

This makes it clear that we are specifying the aspect ratio, and not
the intended display size in pixels.

Originally committed as revision 24239 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/matroskaenc.c

@@ -603,6 +603,7 @@ static int mkv_write_tracks(AVFormatContext *s)
                     int d_width = codec->width*av_q2d(st->sample_aspect_ratio);
                     put_ebml_uint(pb, MATROSKA_ID_VIDEODISPLAYWIDTH , d_width);
                     put_ebml_uint(pb, MATROSKA_ID_VIDEODISPLAYHEIGHT, codec->height);
+                    put_ebml_uint(pb, MATROSKA_ID_VIDEODISPLAYUNIT, 3);
                 }
                 end_ebml_master(pb, subinfo);
                 break;