Added support for tracks with cubic spline interpolation to AnimationUtils.makeClipAdditive

[c-morten]

The current version of this method assumes that a track's valueSize is the number of values the target property can hold (3 for vectors, 4 for quaternions, etc). However for tracks using cubic spline interpolation it is actually 3 times that length to account for in/out tangents, resulting in the in-tangent values being used for the reference value rather than the actual keyframe value. I added a check for the interpolation type and offset the reference and target indices accordingly. I also fixed a small issue in PropertyMixer where the quaternion buffer does not get set to a valid identity quaternion for cubic spline buffers when using PropertyMixer._setAdditiveIdentityQuaternion.

Fixes #20025

[mrdoob]

Thanks!

[FlorentMasson]

Thanks, I spend some hours on this exact issue a week ago