Cycles : Enable OpenImageDenoise + Viewport support

[boberfly]

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

• Adds OpenImageDenoise support back into Cycles, as well as viewport support

Feeling a bit uninspired for how to sketch out a meaningful unit-test, leaving as draft.

Related issues

• NA

Dependencies

• OpenImageDenoise + ISPC added dependencies#239

Breaking changes

• Not breaking but it adds options.useDenoise to the UI

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.