oggstream: implement tag extraction for Kate streams

This will mainly allow Totem to know the language of those streams,
so the subtitle selection menu gets properly filled out.

https://bugzilla.gnome.org/show_bug.cgi?id=638005

ext/ogg/gstoggstream.c

@@ -1758,6 +1758,57 @@ packet_duration_kate (GstOggStream * pad, ogg_packet * packet)
   return duration;
 }
 
+static void
+extract_tags_kate (GstOggStream * pad, ogg_packet * packet)
+{
+  GstTagList *list = NULL;
+
+  if (packet->bytes <= 0)
+    return;
+
+  switch (packet->packet[0]) {
+    case 0x80:{
+      const gchar *canonical;
+      char language[16];
+
+      if (packet->bytes < 64) {
+        GST_WARNING ("Kate ID header packet is less than 64 bytes, ignored");
+        break;
+      }
+
+      /* the language tag is 16 bytes at offset 32, ensure NUL terminator */
+      memcpy (language, packet->packet + 32, 16);
+      language[15] = 0;
+
+      /* language is an ISO 639-1 code or RFC 3066 language code, we
+       * truncate to ISO 639-1 */
+      g_strdelimit (language, NULL, '\0');
+      canonical = gst_tag_get_language_code_iso_639_1 (language);
+      if (canonical) {
+        list = gst_tag_list_new_full (GST_TAG_LANGUAGE_CODE, canonical, NULL);
+      } else {
+        GST_WARNING ("Unknown or invalid language code %s, ignored", language);
+      }
+      break;
+    }
+    case 0x81:
+      tag_list_from_vorbiscomment_packet (packet,
+          (const guint8 *) "\201kate\0\0\0\0", 9, &list);
+      break;
+    default:
+      break;
+  }
+
+  if (list) {
+    if (pad->taglist) {
+      /* ensure the comment packet cannot override the category/language
+         from the identification header */
+      gst_tag_list_insert (pad->taglist, list, GST_TAG_MERGE_KEEP_ALL);
+    } else
+      pad->taglist = list;
+  }
+}
+
 
 /* *INDENT-OFF* */
 /* indent hates our freedoms */
@@ -1903,7 +1954,7 @@ const GstOggMap mappers[] = {
     is_header_count,
     packet_duration_kate,
     NULL,
-    NULL
+    extract_tags_kate
   },
   {
     "BBCD\0", 5, 13,