Mitigates #93726.
With this asset previews will no longer flicker in the asset browser
on actions like undo, redo or when using "Adjust Last Operation" if
the asset library doesn't contain assets from the current file.
Previews of assets from the current file would have to be cleared on
undo & redo, because their memory location can change then. However the
file browser's data model didn't allow clearing just these previews, so
previews from external assets would have to be reloaded from disk too,
causing the flickering.
Noticed we can trivially skip clearing of previews if there are no
assets from the current file anyway. This mitigates the issue quite a
bit until preview handling is rewritten, see #122439.
Julian Eisel
7e99e575a4