Blender Git Loki

Fix T69789: Assert when create a new Full Copy scene base on 2D template.

Private ID data (nodetrees and scene collections...) need special care
and handling of their copy flags, and checks must be adapted too.

In that case, issue came from the fact that even though those IDs have
to be copied outside of bmain, we may still require usercount handling.

That commit also fixes a somewhat related issue - we cannot use the
non-id private data copying flag for private IDs copying, due to
difference in handling of usercount again.