This wasn't working because the code that stores the backup up the current pose
looped over the pose asset's fcurve listbase to determine which bones needed to
be backed up. When the pose asset was stored as a slotted action, that listbase
was always empty, so nothing was backed up. This resulted in exploding poses as
the pose was repeatedly applied on top of itself, and in debug builds would
trigger asserts in code that was sanity-checking quaternion values.
This PR updates that code to loop over the fcurves of the first slot when the
pose asset is stored as a slotted action, which in turn makes pose blending
work as expected with slotted actions.
Pull Request: https://projects.blender.org/blender/blender/pulls/128686
91984fad1b