Add missing LOD.isLOD

src/objects/LOD.js

@@ -26,6 +26,8 @@ LOD.prototype = Object.assign( Object.create( Object3D.prototype ), {
 
 	constructor: LOD,
 
+	isLOD: true,
+
 	copy: function ( source ) {
 
 		Object3D.prototype.copy.call( this, source, false );