Fake User world shader always gets exported

Even if you only select a single object for export using this plug-in, if you have a world shader with a fake user, that gets included with the file (and say, if that world shader happens to have an embedded very large HDRI, the .blend export gets very large).

Start by locating the exporter entry point that gathers data for selected-object exports, then trace how world shaders with fake users are included. Reproduce the issue with one selected object and a fake-user world shader containing an HDRI. Done means the selected-object export no longer includes that unrelated world data, while the intended export behavior remains intact.

Written by the indexing model from the issue text.