Asset::same_file compares pdf_crop (#362) with type, page and bytes: one
PDF page shown with two crop boxes keeps two assets, so add_asset,
adopt_asset, insert_asset (IDML/INDD import) and the load/save merge
never point a placement at another crop. Test in
identical_files_share_one_asset.
The copy clipping (edit.rs clip_doc) keeps the assets used_assets finds
in it, so graphics anchored in copied text, graphic table cells (#379),
notes and media posters keep their files; it collected only graphics on
the copied items before. Test:
copied_text_frames_keep_the_graphics_anchored_in_their_text.