4.0 files now include asset shelf regions in 3D views. This region type is not
known to older Blender versions. So far, in such cases we would just change
the region type to be the first known region type and keep the region storage
otherwise. This was arbitrary, and in fact unsafe: the reused settings may
violate invariants/assumptions for a region type and worse, the
`ARegion.regiondata` can only be interpreted and correctly written to files
if the region type is known.
Make sure all invalid regions (regions where the type cannot be restored) are
removed on file read.
Committed to 3.6 release branch as e2d4403497.
Pull Request: https://projects.blender.org/blender/blender/pulls/111483
Julian Eisel
7fb58a33f0