Animations not written for some keyframe sets

[lasalvavida]

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.