Zero derivs for interactive params when needed

[aconty]

Sometimes an interactive param is used in a way that gets flagged as needing derivs. But we were neither allocating space for that nor zeroing them. This resulted on memory corruption.

Description

This fixes incorrect renders in a production context where params are often tagged as needing derivs.

Checklist:

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

[lgritz]

Hmmm, I think this will work, and I think it's fine to accept. But I wonder if a better fix long-term is that we should be turning derivatives off for a param that is marked as interactive. It won't have non-zero derivs, so any computations done using its derivative are pointless anyway.

[aconty]

I thought about that, but I wasn't familiar with all the moving parts and didn't try.

[lgritz]

I can take a look at that angle tomorrow when I'm back in the office.

If you need a workaround today for builds, it's fine to merge this, it doesn't hurt, I just think we can do better... if we can figure out the right spot to disable the derivs.

[aconty]

We can wait for your take on it tomorrow

[lgritz]

I'm going to merge this now so we can do an internal release, then I will come back to the problem afterwards and see if I can make it disable derivs entirely for these params that are marked interactive. (I don't want to hold up the fix waiting for me to get that right.)