Improve Rhino conversion time on BREP heavy models

[AlanRynne]

This requires scoping first!!! Opened this issue just to start the discussion.

Conversion of very heavy models is always going to be "slow", but in Rhino specifically, where BREPs are the most common geometry you'll find, we could do some improvements in the conversion side to speed up the process:

• Look into multithreading conversion for each object in a layer (similar to how GH does one conversion per object in a task).
• Look into https://developer.rhino3d.com/samples/rhinocommon/extract-render-mesh/. Extracting the existing render mesh would be ideal, as we'd no longer need to re-mesh a BREP you're already seeing on screen (so Rhino has the mesh already somewhere).
• Any other suggestions are welcome!

[AlanRynne]

I guess we forgot to link this to the PR, because we're already doing number 2 in Rhino when available.

If the render mesh is found, we'll skip meshing in favor of the existing one