Add a ShadingSystem::Parameter call variety that can set lockgeom=0
on a particular parameter when the instance values are declared, even if
they were not so marked in their metadata at the time they were compiled
by oslc.

There's an interesting trick involved here: We gather pending parameters
for an instance into a ParamValueList, as a temporary holding area until
the Shader call.  We use the PV's existing 'interp' field to indicate
whether the value has a lockgeom override -- INTERP_CONSTANT as the
default, and anything else (we use INTERP_VERTEX) to mean the value may
vary across the geom, which is what lockgeom is primarily used for.