Both the Follow Curve constraint and parenting with the follow curve option
require a property on the Curve ID to be animated if the user wants the
follower object to animate along the curve. However, the fcurve for this
was always getting created as a legacy action fcurve, regardless of whether
the action was actually legacy or not. If the action was layered, this
resulted in the follower object not animating along the path.
The underlying issue was that the code that created the fcurve was using
`action_fcurve_ensure()`, which hadn't yet been updated to work correctly with
layered actions, and thus the fcurve was created as if the action was legacy.
This commit fixes that by updating `action_fcurve_ensure()` to work with
Baklava phase-1 layered actions in limited circumstances, and also
ensuring that the follow curve code passes an appropriate ID RNA pointer
(required to find/create an appropriate Slot).
Pull Request: https://projects.blender.org/blender/blender/pulls/124353
cb38406a94