ok   wrote the source image
ok   wrote the two-subimage image
ok   ImageBuf: source recorded
ok   ImageBuf: agrees with name and file_format_name
ok   ImageBuf: also on the native spec
ok   ImageBuf, subimage 1: source recorded
ok   ImageCache: source recorded
ok   ImageCache, subimage 1: source recorded
ok   cache-backed ImageBuf: source recorded
ok   ImageBufAlgo result: source kept
ok   ImageBufAlgo result: its own file_format_name is empty
ok   no written file contains the source
done.
