Animations: Use a new mode for "relative from current" animation loop

[Popov72]

Follow up to #14563

It was breaking existing code (see https://forum.babylonjs.com/t/camera-animation-bug/46211 among others), so this PR adds a new loop mode for "relative from current".

[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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