Blender Git Loki

Asset Catalogs: change rules for saving catalog definition files

Change the rules for determining where to save a new catalog definition
file (CDF).

Old situation (T91681): if a `blender_assets.cats.txt` file already
exists in the same directory as the blend file, write to that. If not,
see if the blend file is contained in an asset library, and write to its
top-level CDF.

The new situation swaps the rules: first see if the blend file is
contained in an asset library, and if so write to its top-level CDF. If
not, write a CDF next to the blend file.

As before, any pre-existing CDF is not just bluntly overwritten, but
merged with the in-memory catalogs.