testrender: Implement support for medium closures (volumetric rendering)

[lgritz]

[osomalley]

I've started some initial work in this branch. It's still in its early stages but just wanted to update the thread.

[lgritz]

That's great!

I strongly recommend that you engage here, in email, or on the Slack by explaining at least in broad terms what your design and implementation strategy are, to get some feedback from the many very experienced renderer authors here (who may also have strong opinions about what ends up in testrender) to get feedback before you have invested too much time in a specific implementation.