Fix camera rotation when exporting glTF

[bghgary]

See https://forum.babylonjs.com/t/gltfexport-flipped-camera/45986.

Introduced partially by #13909 but it was already somewhat broken.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

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