smpte: don't register transition types twice

gst/smpte/barboxwipes.c

@@ -948,10 +948,16 @@ static const GstMaskDefinition definitions[] = {
 void
 _gst_barboxwipes_register (void)
 {
+  static gsize id = 0;
+
+  if (g_once_init_enter (&id)) {
     gint i = 0;
 
     while (definitions[i].short_name) {
       _gst_mask_register (&definitions[i]);
       i++;
     }
+
+    g_once_init_leave (&id, 1);
+  }
 }