Non-editable is mostly linked (besides some incoming exception for the
Brush Asset project).
If the Image is linked but not packed, conceptually it does not really
make a difference (edits for a linked vs local image would still end up
in the same external file when saved). However when it is linked and
packed we are running into Undo crashes (#122131). Also the image
remains linked, so data remains to be read/loaded from the packed
version from the library (so any edits are lost).
For consistency, disallow image operations for all non-editable images.
ref. #122131
ref. #122033
Pull Request: https://projects.blender.org/blender/blender/pulls/122209
f93360b3bb