[R] RGL 'front' upwards default in rendering image

[Joe Byers]

The documentation for surface3d and rgl.surface in the package RGL states
"'surface3d' always draws the surface with the `front' upwards
      (i.e. towards higher 'z' values).  This can be used to render the
      top and bottom differently; see 'rgl.material' and the example
      below."

Is there a way to override this default?  I have search all the related 
methods help and the documents on RGL's website.

Thanx
Joe