Cycles : Light-linking

[boberfly]

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

• This adds support for Cycles light-linking, as well as shadow groups (however it is using the Arnold ai:visibility:shadow_group attribute for now until the renderer interface gets updated)

• Should this PR also add a StandardAttributes for shadow groups, now that Cycles supports them and not just Arnold?

• We will also need a shadow-link test but perhaps it should be added as a global test once it is a standard attribute

• I'd love to have new eyes on this as well, as it's a lot of code (however I've been doing some extensive testing and bug-fixing especially for live rendering updates already).

Related issues

• NA

Dependencies

• Cycles : Update to version 4.0.2 #5578

Breaking changes

• CyclesMeshLight now adds those particular meshes to the Lights set so it now gets created like a light, which is needed so that light-linking and shadow groups can work. I've also removed disabling visibility flags as it works differently to Arnold here and they shouldn't be turned off by default (a user can still modify them using a CyclesAttributes if they wish).

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.