IECoreUSD : Maintain support for USD 20

[ivanimanishi]

USD 20 is still used at IE for cortex 10.3, so we need to make sure the code still builds for it.

This was done following @johnhaddon's suggestions.

[ivanimanishi]

Note that we are only adding this to the 10.3 maintenance branch because 10.4 should be using a more modern USD at IE as well.

[johnhaddon]

Note that we are only adding this to the 10.3 maintenance branch because 10.4 should be using a more modern USD at IE as well.

It'll end up on main as well when we do our usual merge forwards. I don't think that should be a problem though?

[ivanimanishi]

Note that we are only adding this to the 10.3 maintenance branch because 10.4 should be using a more modern USD at IE as well.

It'll end up on main as well when we do our usual merge forwards. I don't think that should be a problem though?

I thought that might end up being the case. There is not problem, of course. It's just unnecessary. But if it's easier to keep the change, it's fine.

[johnhaddon]

But if it's easier to keep the change, it's fine.

I can remove it once we've done the merge - it'd be nice to rip out all code for USD versions now the minimum version has moved up for Cortex 10.4. Remind me though, what is the minimum we need to support now? You mentioned that IE is using 22.03 with Cortex 10.4 standalone, but #1226 maintains compatibility with 20.08 for use in Houdini 18.5 at IE. Have you upgraded to Houdini 19 at the same time as Cortex 10.4, in which case the new minimum would be 21.08 (what Houdini 19 uses)?

[johnhaddon]

In #1256 I've played it safe and removed support for everything below 20.08.