Animations goToFrame: Fix breaking change introduced in #15126

[Popov72]

See https://forum.babylonjs.com/t/animation-blending-and-key-interpolation-bug-in-new-cdn/54729/4.

My change in #15126 was a breaking change, because when the weight isn't -1, the animation is considered additive, and this can break if there isn't at least one animation that isn't additive.

I needed this change for Teams Avatar, so I made a change so that the old behavior is the default, but I added a flag so that I could use the animation weight on demand.

[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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[bjsplat]

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[bjsplat]

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[RaananW]

Does this restore the previous behavior?

[Popov72]

Does this restore the previous behavior?

Yes, the new behavior is now under a flag.