WIP: Open OPR - rewrite thin_film layering for compatibility with MDL

[krohmerNV]

In the MDL code generator we currently can't handle layering constructs like this: layer(mix(bsdf1, bsdf2, weight), bsdf3) so I propose a workaround that should not change the result but uses an alternative implementation: mix(layer(bsdf1, bsdf3), layer(bsdf2, bsdf3), weight)