libavformat: Add generic disc_subtitle tag mapping for Vorbis

The mapping is taken from Picard's [1] and taglib's [2] documentation.

The official list [3] of standard field names is pretty short and
does not include one for a disc/part subtitle. Some taggers use
SETSUBTITLE here instead. However, I think DISCSUBTITLE is much more
consistent with the already existing DISCNUMBER field and it matches
what Picard and taglib do.

[1]: https://picard-docs.musicbrainz.org/en/appendices/tag_mapping.html#disc-subtitle
[2]: https://taglib.org/api/p_propertymapping.html
[3]: https://xiph.org/vorbis/doc/v-comment.html

libavformat/vorbiscomment.c

@@ -32,10 +32,12 @@
  * http://xiph.org/vorbis/doc/v-comment.html
  */
 const AVMetadataConv ff_vorbiscomment_metadata_conv[] = {
-    { "ALBUMARTIST", "album_artist"},
-    { "TRACKNUMBER", "track"  },
-    { "DISCNUMBER",  "disc"   },
-    { "DESCRIPTION", "comment" },
+    { "ALBUMARTIST",  "album_artist"  },
+    { "TRACKNUMBER",  "track"         },
+    { "DISCNUMBER",   "disc"          },
+    /* Adapted from <https://picard-docs.musicbrainz.org/en/appendices/tag_mapping.html#disc-subtitle> */
+    { "DISCSUBTITLE", "disc_subtitle" },
+    { "DESCRIPTION",  "comment"       },
     { 0 }
 };
 

tests/ref/fate/generic-tags-remux-vorbiscomment

@@ -1,5 +1,5 @@
-1d3dcf6e56ee623bcee8bc2d87098a9d *tests/data/fate/generic-tags-remux-vorbiscomment.flac
-8647 tests/data/fate/generic-tags-remux-vorbiscomment.flac
+e9e51f486ae8907794c46377678c5203 *tests/data/fate/generic-tags-remux-vorbiscomment.flac
+8646 tests/data/fate/generic-tags-remux-vorbiscomment.flac
 #extradata 0:       34, 0x93a20b79
 #tb 0: 1/44100
 #media_type 0: audio