ShadingSystem::ReParameter() lets the app send a new instance param value
after the shader group has been declared, if the group has not yet been
optimized OR if the param was marked lockgeom=0.

An interesting use of this is in an interactive system that allows
adjustment of shader parameters.  If you mark the parameters that you
wish to adjust as 'lockgeom=0' (via the original Parameter declaration),
then you can use ReParameter() to replace the parameter instance value
and rerender immediately, without needing to re-declare and re-compile
the shader network.  It will not have turned the parameter value into
a constant when optimizing, since it was marked as lockgeom=0.