Treat the "mper" attribute as the 64-bit item that it is rather than a 32-bit item as hithereto. Output it as a hexadecimal number to correspond with the format of the track id obtained from AppleScript

mpris-service.c

@@ -25,6 +25,7 @@
  */
 #include <stdio.h>
 #include <string.h>
+#include <inttypes.h>
 
 #include "config.h"
 
@@ -174,8 +175,7 @@ void mpris_metadata_watcher(struct metadata_bundle *argc, __attribute__((unused)
   // Add in the Track ID based on the 'mper' metadata if it is non-zero
   if (argc->item_id != 0) {
     char trackidstring[128];
-    snprintf(trackidstring, sizeof(trackidstring), "/org/gnome/ShairportSync/mper_%u",
-             argc->item_id);
+    snprintf(trackidstring, sizeof(trackidstring), "/org/gnome/ShairportSync/%" PRIX64 "", argc->item_id);
     GVariant *trackid = g_variant_new("o", trackidstring);
     g_variant_builder_add(dict_builder, "{sv}", "mpris:trackid", trackid);
   }