barriers: Free backend implementation at dispose time

When a MetaBarrier is first created it allocates a backend
impl object which does the actual heavy lifting.

Unfortunately, that backend object is never freed.

This commit ensures the implementation gets freed when
the barrier object is freed.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1451

src/backends/meta-barrier.c

@@ -128,6 +128,7 @@ static void
 meta_barrier_dispose (GObject *object)
 {
   MetaBarrier *barrier = META_BARRIER (object);
+  MetaBarrierPrivate *priv = barrier->priv;
 
   if (meta_barrier_is_active (barrier))
     {
@@ -135,6 +136,8 @@ meta_barrier_dispose (GObject *object)
                 barrier);
     }
 
+  g_clear_object (&priv->impl);
+
   G_OBJECT_CLASS (meta_barrier_parent_class)->dispose (object);
 }