Backport fixes for CVE-2025-64181 etc. in OpenEXRCore

[musicinmybrain]

Description of Change(s)

This PR attempts to backport fixes for CVE-2025-64181 / GHSA-3h9h-qfvw-98hq and other issues from OpenEXR 3.4.3.

I have made a reasonable effort to backport these intelligently, but it’s not easy for me to test this PR properly other than confirming that it still compiles.

It might be even better to rebase the bundled and modified copy of OpenEXRCore (as described in #2619) on a later version, but that’s far beyond what I can reasonably offer in a PR.

Link to proposal (if applicable)

N/A

Fixes Issue(s)

Checklist

• I have created this PR based on the dev branch

• I have followed the coding conventions

• I have added unit tests that exercise this functionality (Reference:
  testing guidelines)

N/A, no new functionality

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

N/A; it is hard for me to run these locally

• I have submitted a signed Contributor License Agreement (Reference:
  Contributor License Agreement instructions)

[jesschimein]

Filed as internal issue #USD-11678

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