VREffect: unset the rendertarget before clear (#9695)

This will unset the current rendertarget of the renderer if no
rendertarget is specified to the render()-call.

examples/js/effects/VREffect.js

@@ -281,6 +281,7 @@ THREE.VREffect = function ( renderer, onError ) {
 
 			} else  {
 
+				renderer.setRenderTarget( null );
 				renderer.setScissorTest( true );
 
 			}