Apple: Enable iOS Imaging support

[dgovil]

Description of Change(s)

This PR ports Thor's imaging changes against the latest dev. This should enable imaging on all iOS derivatives, including visionOS. I think I got everything, and it does build/run for me, but as mentioned I haven't done significant testing just yet.

I think we can use this branch to iterate the changes for imaging as needed.

Fixes Issue(s)

• [ ] I have verified that all unit tests pass with the proposed changes

• [X] I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-9841

[jesschimein]

/AzurePipelines run

[azure-pipelines[bot]]

Azure Pipelines successfully started running 1 pipeline(s).

[dgovil]

I need to fix up some issues the arose from rebasing. Fixing it up now, and should have it good to go soon.

[dgovil]

With some of the changes in 24.8, I'm just going to close this PR and re-open it later once I can resolve some things. Seems like its a little more involved than I thought.

[dgovil]

Okay that actually was less involved than I thought. It required a patch to MaterialX (thanks Maddy, and I'll look into mainlining it) and some linker changes.

I'll resubmit this PR because it won't let me re-open this for some reason.

[dgovil]

Re-opened here https://github.com/PixarAnimationStudios/OpenUSD/pull/3215