Update Imath to 3.1.12, openexr 3.3.2

[TodicaIonut]

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PR Description

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
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What does this pull request do?

Updated Imath package to the latest stable version (3.1.12).

[TodicaIonut]

@acolwell Update openexr?

[acolwell]

@acolwell Update openexr?

Sure. Just a few things.

1. Please rebase onto the current RB-2.6 so you get the windows installer fixes checked in earlier today.
2. Please update tools/MINGW-packages/windows_pacman_repo_version.txt to 20241220-1 so that the builds will use your new version of the repo.
3. Please run the Build Windows Installer action on these changes in your repro and post the link to the successful run here so we can be sure the tests still pass and the installer still builds.