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