Animations not written for some keyframe sets

This one particularly: https://github.com/assimp/assimp/blob/master/test/models/Collada/library_animation_clips.dae

When multiple animations target the same node, we merge the animation keyframes and interpolate missing values, but this isn't really possible for matrix transform types.

In this model however, it looks like the animations do share the same keyframes, some are just longer than others, but the failure gets triggered anyway. This is a bug.

On a side note, in the short term, a warning should be emitted if this happens, or more ideally shouldn't happen at all if possible. It's a hold over from how things were done in the old converter and there's probably a more elegant way to handle this.

Start with the referenced test/models/Collada/library_animation_clips.dae file and trace the animation keyframe merging and interpolation path for multiple animations targeting one node. Compare the shared and differing keyframe lengths, then verify that exporting this model no longer triggers the failure; the issue also identifies a warning as a short-term fallback.

Written by the indexing model from the issue text.