avformat/matroskaenc: don't write DisplayUnit with value Unknown on WebM files

Value 4 (Unknown) is for the time being part of the Matroska spec but not
supported by WebM

Addresses ticket #6176

libavformat/matroskaenc.c

@@ -1258,7 +1258,7 @@ static int mkv_write_track(AVFormatContext *s, MatroskaMuxContext *mkv,
         } else if (display_width_div != 1 || display_height_div != 1) {
             put_ebml_uint(pb, MATROSKA_ID_VIDEODISPLAYWIDTH , par->width / display_width_div);
             put_ebml_uint(pb, MATROSKA_ID_VIDEODISPLAYHEIGHT, par->height / display_height_div);
-        } else
+        } else if (mkv->mode != MODE_WEBM)
             put_ebml_uint(pb, MATROSKA_ID_VIDEODISPLAYUNIT, MATROSKA_VIDEO_DISPLAYUNIT_UNKNOWN);
 
         if (par->codec_id == AV_CODEC_ID_RAWVIDEO) {