Fix MeshInstance3D not reattaching skeleton after switching World3D

[adamwych]

VisualInstance3D detaches the skeleton after receiving NOTIFICATION_EXIT_WORLD:

godot/scene/3d/visual_instance_3d.cpp

Lines 108 to 112 in 923c751

	case NOTIFICATION_EXIT_WORLD: {
	RenderingServer::get_singleton()->instance_set_scenario(instance, RID());
	RenderingServer::get_singleton()->instance_attach_skeleton(instance, RID());
	_set_vi_visible(false);
	} break;

But MeshInstance3D attaches it only when it enters the tree, not if the world is changed.