Call render() in loader callback

As discussed in https://github.com/mrdoob/three.js/pull/14957#issuecomment-424184966.

examples/webgl2_materials_texture3d_volume.html

@@ -159,6 +159,8 @@
 
 				var mesh = new THREE.Mesh( geometry, material );
 				scene.add( mesh );
+				
+				render();
 
 			} );