7fcb565776
Caused by 6d832ee5b3
The bone names were flipped correctly, however, we were missing the
actual FCurve BezTriple flipping.
This was caused by passing the wrong FCurve into
`do_curve_mirror_flippping` -- this function is checking for existence
of the FCurve in the `KeyframeCopyBuffer`, so we gotta pass
`fcurve_in_copy_buffer` (instead of the new fcu).
Pull Request: https://projects.blender.org/blender/blender/pulls/135381