option for using or not using geom's indices for tangent space vectors
generation. now, Ruben's code is also used (it was disabled before). in
order to keep backward compatibility, the compute method behaves as before
in default case."
Note from Robert. New codes produces errors when running
osgfxbrowser cow.osg so I have elected to keep the original
implementation of compute_basis_vectors around and as the default
compile to keep the CVS version working. Ruben's new version
of compute_basis_vectors has to be explicitly compiled in by
toggling an #if 1 block to #if 0.
