title: Plot multiple 3D images in a single plot window with plot3d
body: <p>When I plot several 3D images using <code>plot3d</code> from the <code>rgl</code> package, the images are displayed separately. I want to show them in one plot, as when using, e.g., <code>par(mfrow=c(2, 2))</code> to display four 2D images in a single plot window. </p>
  
  <p>Is this possible?</p>