Skeleton3D: Convert 3.x skeletons

[nikitalita]

This implements conversion of the old 3.x Skeleton format to 4.x upon load (similar to old Mesh format conversion).

Skeletons had a Transform pose property in 3.x, which was broken out in 4.x to

• Vector3 pose_position;
• Quaternion pose_rotation;
• Vector3 pose_scale

Additionally, in 3.x, the pose of the skeleton was relative to the rest position; in 4.x, the poses are absolute.

So, we recompute the poses to be absolute and break the Transform3D into its various components.

[fire]

I think this is good. @TokageItLab @lyuma Can you check?

This was how it was described to me by Tokage YEARS ago.

[TokageItLab]

This should be fine for the bone properties.

However, there is no animation compatibility, so if you want full compatibility with 3.x Skeleton, you will need to consider that as well; There is never full compatibility possible with animations. Bone animation is now absolute, not relative, and scale has been isolated so that transforms should be different when combined with rotation and scale.

[nikitalita]

This should be fine for the bone properties.

However, there is no animation compatibility, so if you want full compatibility with 3.x Skeleton, you will need to consider that as well; There is never full compatibility possible with animations. Bone animation is now absolute, not relative, and scale has been isolated so that transforms should be different when combined with rotation and scale.

Yes, this is fixed in my next PR. #87106

[YuriSizov]

Thanks!