28f84831c3
Th compositor asserts when loading certain files. Those file were saved
with an invalid active viewer key, possibly due to a bug that was fixed
already in c8cb24121f, but blend files were never updated.
So starting in 5.1, we fix those files by essentially doing what
ED_node_set_active_viewer_key is supposed to do at load time during
versioning.
Pull Request: https://projects.blender.org/blender/blender/pulls/148047