Update when the skeleton skin is changed.

scene/3d/skeleton.cpp

@@ -770,6 +770,7 @@ Ref<SkinReference> Skeleton::register_skin(const Ref<Skin> &p_skin) {
 	skin_bindings.insert(skin_ref.operator->());
 
 	skin->connect("changed", skin_ref.operator->(), "_skin_changed");
+	_make_dirty();
 	return skin_ref;
 }