Changing a node tree item property (such as the default value) was using a very
broad and generic "tag" function which invalidates the runtime items cache.
This also invalidates any python iterators due to the API using the runtime
cache. Changing node tree items in a loop will then crash.
It's not necessary to invalidate the runtime items cache when the actual item
pointers have not changed. Most RNA updates only change superficial properties,
or at most require a recursive node tree update due to change of identifiers or
types.
This PR introduces a simpler "tag" function to only tag for tree updates by not
rebuild the entire runtime items cache. It also renames existing functions and
docstrings to better explain what each of them does and should be used for.
The `NodeTreeInterfaceChangedFlag` is removed completely because it is only ever
used as a simple boolean indicator of "item changes" that require a cache
rebuild. It is replaced with an atomic bool like flags used for runtime caches.
Pull Request: https://projects.blender.org/blender/blender/pulls/143932
dab6b45336