A modification that holds and executes a [SkeletonModificationStack3D]. This [SkeletonModification3D] holds a reference to a [SkeletonModificationStack3D], allowing you to use multiple modification stacks on a single [Skeleton3D]. [b]Note[/b]: The modifications in the held [SkeletonModificationStack3D] will only be executed if their execution mode matches the execution mode of the SkeletonModification3DStackHolder. Returns the [SkeletonModificationStack3D] that this modification is holding. Sets the [SkeletonModificationStack3D] that this modification is holding. This modification stack will then be executed when this modification is executed.