/// MaterialInfoTable::material_info() takes a position object as input,
/// computes or looks up a MaterialInfo object, and returns a pointer to it.
/// If the material configuration is not already present in the table, it
/// MaterialInfoTable::material_info() takes a position object as input,
/// computes or looks up a MaterialInfo object, and returns a pointer to it.
/// If the material configuration is not already present in the table, it