store image without visualizer output

    vis = o3d.visualization.Visualizer()
    vis.create_window()
    vis.add_geometry(pcd)
    vis.update_geometry()
    vis.poll_events()
    vis.update_renderer()
    vis.capture_screen_image(path)
    vis.destroy_window()
    https://github.com/intel-isl/Open3D/issues/1095