Blender Git Loki

Fix T51955: Changing Auto-IK length crashes Blender (with new-depsgraph)

New dependency graph is tacking root bone into account when building the graph.
This is required in order to get proper dependencies between bones. so we can
reliably use bones as targets from the same rig (and even indirect relations
via external objects). This forces us to tag relations for update when we change
root IK chain bone.

Since relations rebuild is not fully trivial operation, we only do it for
the new dependency graph. In the future it'll be nice to avoid whole graph
rebuild for such cases, but that's mentioned as a TODO.