Fixed dynamic cube map cars demo.

Finally, the trouble was at the application level: renderer.clear clears the current framebuffer, which, if you juggle multiple render targets, doesn't have to be necessarily the screen. Here it was the last face of cube render target.

examples/webgl_materials_cubemap_dynamic.html

@@ -1012,7 +1012,7 @@
 
 				camera.lookAt( cameraTarget );
 
-				renderer.clear();
+				renderer.clearTarget( null );
 				composer.render( 0.1 );
 
 				renderer.shadowMapEnabled = false;