Port Windows fix for unregistering OgreGLWindows class in ogre 2.2

[iche033]

🦟 Bug fix

Summary

Unregister window class on Windows to allow repeated loading / unloading of ogre 2.2 (used by ign-rendering6) in the same process, i.e. our unit and integration tests. A similar patch (OGRECave/ogre-next#327) was already submitted by @mjcarroll to ogre-next for version 2.3 (used by gz-rendering7 onwards). Adding the change here for ign-rendering6 so we can run ogre2 tests on our new Windows conda CI.

Note that the gz_rendering-pr-win jenkins job in the checks below still uses the old vcpkg based builds so we'll still get many test failures.

Here are the new conda test builds:

• before: 59 test failures - https://build.osrfoundation.org/view/conda/job/_test_gz_rendering-pr-cwin/68/
• after: 8 test failures - https://build.osrfoundation.org/view/conda/job/_test_gz_rendering-pr-cwin/71/

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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