The issue here is that motion_steps handling is a bit complex, and the
parallel synchronization of geometry does not play well with it.
The obvious result of this was a crash related to the main thread
checking attributes while the geometry sync was changing them, but
there was also another race condition that could result in ending up
with the wrong motion_steps.
Specific changes:
- Change place where `motion_steps` is set to avoid concurrent access
- Change the default `motion_steps` to zero, since they won't be
explicitly set if there's no motion now
- Don't skip `motion_steps` copy in `sync_X` since it's no longer set
in `sync_object` and we need to transfer the value in case it was set
to 3 by the velocity code since that's no longer the default
Pull Request: https://projects.blender.org/blender/blender/pulls/133669
ace85d3338