oiiotool: make --fit:exact=1 work for all subimages (#1676)
oiiotool: make --fit:exact=1 work for all subimages (#1676)

It was just fitting the first subimage and dropping the others on the floor.
lgritz/oiio,OpenImageIO/oiio,OpenImageIO/oiio,lgritz/oiio,OpenImageIO/oiio,lgritz/oiio,OpenImageIO/oiio,lgritz/oiio