Additive animation does not always evaluate properly with cubic spline interpolation

[c-morten]

Description of the problem

When converting an AnimationClip to additive using AnimationUtils.makeClipAdditive when the clip contains animation with cubic spline interpolation, the tangent values get subtracted instead of the actual keyframe value. I submitted a PR with a fix for the issue here: #20021

To reproduce:

• Pull the repository
• Replace examples/models/gltf/xbot.glb with the glb from the attached zip file
  xbot.zip
• Do npm run dev and open one of the server links
• Navigate to the animation/skinning/additive/blending example
• Set any of the Additive Action Weights to a non-zero value

Expected

The character blends the additive animation on top of the animations in Base Actions

Actual

The character disappears

Three.js version

• Dev
• r119
• ...

Browser

• All of them
• Chrome
• Firefox
• Internet Explorer

OS

• All of them
• Windows
• macOS
• Linux
• Android
• iOS

Hardware Requirements (graphics card, VR Device, ...)