Adding "writeback" callbacks to the depsgraph should be temporary, since these
callbacks can bind pointers that become invalid. In this case: the evaluated
nodes modifier bound by a bake node callback. These callbacks are cleared after
being added in the `deg_flush_updates_and_refresh` function, but there are other
cases where depsgraph updates are executed which don't support writeback
callbacks (`object_force_modifier_update_for_bind` run by the smooth modifier
"bind" function).
To prevent dangling invalid pointers in outdated callbacks, disallow adding
callbacks in any case other than the `deg_flush_updates_and_refresh` function.
Since callbacks can be added from any depsgraph update, the safest way to
prevent adding them is in the depsgraph itself. If the `use_writeback_callbacks`
flag is not set, any callback is simply discarded.
Pull Request: https://projects.blender.org/blender/blender/pulls/137083
10e95bebca