Blender Git Loki

Fix T59622: dependency problems with Spline IK.

The bug is caused by problems in the dependency graph. Unfortunately,
fixing is not just a matter of fixing the graph, because correct
dependencies would cause a cycle here and in other reasonable use
cases. The real fix thus requires refactoring Spline IK to require
curve data only in the actual evaluation phase, and not in POSE_INIT_IK.

In addition, this separates the normal bone evaluation loop from
Spline IK computations for two reasons:

- That still needs to be done even if spline IK can't evaluate
due to missing curve data.

- It should reduce issues with induced shearing, as Spline IK now
controls how parent-child relations are handled in the chain, and
can take care to only carry over rotation and location.