FBXLoader: Fix rotation discontinuities.

[solinehayes]

Fix #27056

Description
This PR addresses an issue in the FBXLoader where rotations with significant differences between consecutive frames were interpolated using Euler angles (each coordinate separately), which could introduce discontinuities. The proposed solution detects and corrects these discontinuities by using quaternion interpolation (taking into account possible unroll/quaternion negation), ensuring smooth rotation.

[Mugen87]

I can confirm that your PR fixes the issue! Great! 🎉

Still it's important to review the new code a bit more closely so let's target r159 for this change.

Side note: The maintenance of FBXLoader is currently a bit tricky since no one of the current active collaborators works at the loader. I have implemented and reviewed a few smaller changes for the loader in the recent past but I'm definitely no FBX expert 😅 . So this PR might take a little bit longer to get reviewed.

I wish we had an additional dev with FBX knowledge who could help out reviewing FBX related PRs.

[WestLangley]

The maintenance of FBXLoader is currently a bit tricky

Agreed.

@solinehayes Out of curiosity, is using GLB format an acceptable alternate workflow for you -- or is there a particular advantage to FBX format?

[solinehayes]

The maintenance of FBXLoader is currently a bit tricky

Agreed.

@solinehayes Out of curiosity, is using GLB format an acceptable alternate workflow for you -- or is there a particular advantage to FBX format?

Well, I am working in a company where we're using Maya, so we majorly use FBX. We occasionally use GLB but we prefer FBX.
Plus we're working with animation and GLB only allows multiple animations in one asset, we can not animate several character with several animations.