Inconsistent type assumption for scratch variable

[javagl]

^{(I originally mentioned this in #11665, but should probably be an own issue)}

The scratchVariable in ModelAnimationChannel is initialized depending on the "path" of the channel: For "translation" animations, it is a Cartesian3, and for "rotation" animations, it is a Quaternion. But ... it is a global variable 😬 When initializing multiple channels, then there are cases where this should be a Quaternion, but it actually is a Cartesian3, meaning that some of the functions are just shoving some .w property into that poor Cartesian3. It happens to work (hooray to untyped languages), but ... feels wrong.

Would the preferred way of fixing this be to convert that scratchVariable an instance variable in the ModelAnimationChannel?

[ggetz]

I think the preferred solution would be to have two scratch variables, one for each type.