Blender Git Loki

Fix T81218: Crash in pose mode using a driver on bendy bone Segment

The example file in T81218 has a driver that maps a bone's X-location to the number of BBone segments. This caused a dependency cycle, which resulted in bad thread serialisation, which caused the crash.

This patch breaks the dependency cycle `BONE_LOCAL` ? `DRIVER(bones["Bone"].bbone_segments)` ? `BONE_LOCAL`. The 'Driver Data' relation now points to `BONE_SEGMENTS` when the driven property starts with `bbone_`.

Differential Revision: https://developer.blender.org/D9122