Cycles : Add opacityMode support for convertUSDShaders

[boberfly]

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

• USDPreviewSurface has an opacityMode https://openusd.org/release/spec_usdpreviewsurface.html which states that transparent acts more like a refractive surface eg. specular isn't affected and is described as "glass". The presence mode acts like it does normally with changing alpha on the principled_bsdf.
• If opacityThreshold is being used, the cut-out behaviour also works for transparent mode. This is a little vague, I'm going by their description of the spec here but might need another pair of eyes...
• Fixes ALab's lab beaker/flask equipment to be refractive instead of invisible.

Related issues

• NA

Dependencies

• NA

Breaking changes

• Because transparent is considered default, assets which rely on the old behaviour will explicitly need to set opacityMode to presence.

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]

Murray pointed out that the normals stuff requires Cycles 4.5, which we don't use for Gaffer 1.6. Do you want to retarget this all to main (where we're using Cycles 5.0), or shall we merge the opacity to 1.6_maintenance in isolation and then let you retarget the normals only to main?

[boberfly]

Murray pointed out that the normals stuff requires Cycles 4.5, which we don't use for Gaffer 1.6. Do you want to retarget this all to main (where we're using Cycles 5.0), or shall we merge the opacity to 1.6_maintenance in isolation and then let you retarget the normals only to main?

Oh right for some reason I was thinking it was Cycles 4.5 in Gaffer 1.6 - yep that sounds like a good plan, the opacity stuff was in a separate commit I can do the normals one just for main.