Filter primvars not used by the render

**Describe the feature you'd like**
On productions scene it's not uncommon to see prim with many primvars coming from simulations or used for skinning, those primvars don't contribute to the final render but are passed to Arnold and consume extra memory. One example is the skel:* parameters that are passed to arnold and not used in the render.

**Describe the solution you'd like**
We would like to pass only the attributes needed for the render, to reduce the memory used by Arnold. One idea could be to scan for the primvars used by the shaders and keep only those.

No files, tests, or entry points are named. Start by tracing how primvars, including skel:* parameters, are passed to Arnold and how shaders identify the attributes needed for rendering; done means unused primvars are excluded without affecting the final render and memory use is reduced.

Written by the indexing model from the issue text.