Oops, it was possible after saving an outfit to get into a state where we would show the `<OutfitThumbnailIfCached />` behind the outfit even after it was saved, and then removing items would look weird until auto-saving caught up.
We had used the `backdrop` property because we wanted smoother partial load-ins, but for now I'm just fixing this by switching it to `placeholder`, which already has the right loading-only behavior.
This was also the only call site for `backdrop`, so I've removed it!
