Shaders exports should be placed under a Material prim

[sebastienblor]

When shaders are exported (alone) from arnold plugins, the root shader has a user data "material_surface" giving the name of the Material in the DCC. Same for shaders used as displacement with a user data "material_displacement". This is what we have in the Arnold scene description.

We should make it so that our writer puts such shaders under a Material primitive. This way the shaders can be rendered through hydra

[autodesk-oss-arnold-bot[bot]]

Issue synced internally to ARNOLD-15337