Blender Git Loki

ID management: Do not assume that `NO_MAIN` means `NO_USER_REFCOUNT`

While this is still very fuzzy in current code, this old behavior makes
it close to impossible to efficiently use out-of-main temp data, as it
implies that we'd need to update refcounts everytime we add something
back into BMain (an 'un-refcount' ID usages when removing from BMain).

Now that we have two separate flags/tags for those two different things,
let's not merge them anymore.

Note that this is somewhat on-going process, still needs more checks and
cleanup. Related to T88555.