Test interactive parameter initialization in optix testrender

[chellmuth]

Description

This change updates testrender's optix implementation of reparameter, and adds a new test.

Previously we were grabbing the pointers from incorrect SBT records, so this moves them into a global lookup table.

Tests

I don't know how to test that parameter updates work correctly, but this at least validates that initial values are correct.

Checklist:

• [x] I have read the contribution guidelines.
• [x] I have previously submitted a Contributor License Agreement.
• [x] I have updated the documentation, if applicable.
• [x] I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
• [x] My code follows the prevailing code style of this project.

[aconty]

LGTM!

[lgritz]

I don't want you to bother changing this PR, but just noting that technically, although this is testing that the values you specify for parameters marked "interactive" make it to the shader, I believe it's never calling ReParameter and verifying that you can change the value after the shader has run, and then execute it again to see use of the new value. (That would be a somewhat bigger change to testrender, and probably not worth our time at the moment, since we do test it via testshade.)