3Delight camera overscan support

[gkocov]

Generally describe what this PR will do, and why it is needed

• Camera overscan support (StandardOptions / Overscan ... ) for 3Delight

This is my first Gaffer pull request, so I hope I have followed the expected conventions and processes correctly. If all is well with this pull request, I have several additional improvements for 3Delight's support in Gaffer that I have already implemented and I'll create individual pull requests for each of these additional features.

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Gaffer project's prevailing coding style and conventions.

[johnhaddon]

Thanks Goran, and congrats on your first full request! Gaffer's 3Delight support has always lagged well behind our Arnold and Cycles support, so its brilliant to see some progress on this front - I'm looking forward to your other PRs too.

I've made a couple of comments inline that should hopefully be easy to address. It would also be great if you could add a unit test for your new functionality - you can see the current camera test here :
https://github.com/GafferHQ/gaffer/blob/main/python/IECoreDelightTest/RendererTest.py#L445.

Thanks!
Cheers...
John

[gkocov]

Thanks for the kind words John! I updated the overscan code and comment per your suggestions and I also added the overscan unit test. If there are any additional changes needed please let me know. I'll start on the other PRs in the mean time.

[johnhaddon]

Thanks for the update! I've rebased to fix the merge conflict in Changes.md, but otherwise this is unchanged. Merging!