Previously, whenever the zone detection algorithm could not find a result, zones
were just not drawn at all. This can be very confusing because it's not
necessarily obvious that something is wrong in this case.
Now, invalid zones and links that made them invalid have an error.
Note, we can't generally detect the "valid part" of zones when there are invalid
links, because it's ambiguous which links are valid. However, the solution here
is to remember the last valid zones, and to look at which links would invalidate
those. Since the zone-detection results in runtime-only data currently, the
error won't show when reopening the file for now.
Implementation wise, this works by keeping a potentially outdated version of the
last valid zones around, even when the zone detection failed. For that to work,
I had to change some node pointers to node identifiers in the zone structs, so
that it is safe to access them even if the nodes have been removed.
Pull Request: https://projects.blender.org/blender/blender/pulls/139044
f3294bbd06