When writing a layered Action to disk, take the F-Curves from the
first keyframe strip and write that as `action.curves` as well. This
will make older Blender versions see those curves and load them
properly.
Only the curves for the first slot are written this way. This means
that any legacy Action that was converted to a layered Action will be
loaded again properly by older Blender versions. Of course this is
limited to a single layer, single strip, and single slot -- once the
newer features are used, older versions of Blender will not be able to
see this extra data.
When an Action contains multiple slots, so with animation for multiple
distinct objects, the forward compatibility becomes a bit iffy. Older
versions of Blender will just see a legacy Action, with its legacy
semantics, and thus all objects that use that Action will receive the
exact same animation data. I don't think there's a way around this.
(_Unless we start breaking up Actions into an Action per slot, alter
the assignments, and then store metadata so that modern Blenders can
reassemble them. I do not think this is a good idea._)
Ref: #124714
Pull Request: https://projects.blender.org/blender/blender/pulls/125065
c7bf1a697e