This avoid having to flush explicitely and the need for syncing.
It also removes a lot of complexity in the process.
These updates are not granular and do not need to so much
boiler plate code.
The depsgraph update counter now becomes atomic to avoid
undefined behavior when a depsgraph is being destroyed and
its memory reused (same thinking as the non-copy-on-eval IDs).
I tested some use cases (object update, sculpt update,
shading update) and they are all working.
Pull Request: https://projects.blender.org/blender/blender/pulls/135580
2131ef0a20