Find target using get_parent to support reparenting at runtime.

Previously this was using get_parent_node_3d(), which returns null if
top_level=true, which it is. Thus this check is guaranteed to fail except
right when the Smoothing node is first initializing.

Closes lawnjelly#58.

addons/smoothing/smoothing.gd

@@ -141,8 +141,8 @@ func _FindTarget():
 	# If no target has been assigned in the property,
 	# default to using the parent as the target.
 	if target.is_empty():
-		var parent = get_parent_node_3d()
-		if parent:
+		var parent = get_parent()
+		if parent and parent is Node3D:
 			_m_Target = parent
 		return