Meta-issue: testrender wish list

# Master list of testrender feature requests

We will use this issue to collect individual suggestions or feature requests for testrender. Some are larger projects that can be later forked off to their own issues, but some are as small as single-day tasks for newcomers. All the individual tasks will be added as sub-issues (if we can figure out how to do that).

Most of these testrender feture suggestions don't really require knowledge of how OSL itself is implemented under the covers, but having written a student project sized path tracer might be helpful (since that's basically what testrender is).

Please comment/reply on this ticket to request that items be added (or if you're on the TSC or have permissions to edit this description, feel free to do so. Or add "sub-issues."

Comment/reply on the individual sub-issues to say "hey, I want to do task X" so that multiple don't independently attempt the same tasks (if it's something where only one can really be accepted); we don't want anybody's work time to be wasted.

Small tasks ideal for newcomers to the project, suitable for Dev Days, should have their sub-issues tagged as "good first issue".

This is a master wishlist rather than a scoped implementation task. Review the testrender suggestions collected here, choose a separately defined sub-issue, and follow that item's entry points and acceptance criteria; this issue is complete when the request is split into actionable tasks or the relevant sub-issue is implemented.

Written by the indexing model from the issue text.