The merge down operator was sometimes copying the wrong frame, which altered the animation.
While merging the layers, it is sometimes needed to duplicate a keyframe,
when the lowest layer does not have a keyframe but the highest layer does.
Instead of duplicating the previous keyframe of the lowest layer, the code
was actually duplicating the active frame of the layer which was the current frame in the timeline.
This patch fixes the issue by setting the previous keyframe of the layer as its active frame before duplication.
Related issue: T104371.
Differential Revision: https://developer.blender.org/D17214
Amelie Fondevilla
6d297c35c8