Refactor MDL graph for OpenPBR to avoid layered mixes (#2215)

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)`

Author: krohmerNV