Fixes #11685

It is not really "required", and does not solve "problem" (except for that itchy feeling in the head of people who usually use typed languages): The scratchVariable that was used either as a Cartesian3 or as a Quaternion has now been split into a scratchCartesian3 and scratchQuaternion that are used depending on the animated path (i.e. the cartesian for "translation" and "scale", and the quaternion for the "rotation").

Should this be mentioned in CHANGES.md? It seems like something that is "too internal" to be mentioned...

I considered to add a unit test - basically consisting of

    // Create rotation and translation (leaving the scratch variable to be a Cartesian3)
    const runtimeChannelRotation = new ModelAnimationChannel({.,.});
    const runtimeChannelTranslation = new ModelAnimationChannel({...});
...
    // Animate rotation (using the Cartesian3 scratch variable...)
    runtimeChannelRotation.animate(time);

But none of that is "visible" at the outside: The runtimeNode["rotation"] is still a Quaternion, because the setter in ModelRuntimeNode clones the input value - which is a Cartesian3, but it does have that .w component, so ... it happens to work 😬 If anyone has an idea for how to "test" this, I'd add the tests accordingly...