Support for Baking

[Vochsel]

Unknown if this works with the USD/Hydra pardigm. It seems like we can support it through render settings and render products. But could produce strange results for interactive renders in Houdini.

Started some work here: https://github.com/tangent-opensource/hdcycles/tree/feature/baking

I've added the appropriate schema to usdCycles for the next release. No harm in adding it for future use. But if this is deemed impossible, we should probably remove it