dcb9312687
Unused no-op operation nodes are not bound to a callback function, and
have no outgoing relations. Incoming relations of such nodes are removed
since ff60dd8b18. However, this was done
too broadly, causing too many relations to be lost and indirectly linked
objects to be unevaluated.
This commit introduces a `DEPSOP_FLAG_FAKE_USER` flag for operation
nodes, which indicates they are not to be removed, even when they appear
to be unused.
Reviewed By: sergey
Differential Revision: https://developer.blender.org/D7074