iconvert: fix to handle multi-image files that can't append subimages
iconvert: fix to handle multi-image files that can't append subimages

cwilling/oiio,OpenImageIO/oiio,sambler/oiio,lgritz/oiio,bdeluca/oiio,OpenImageIO/oiio,scott-wilson/oiio,micler/oiio,bdeluca/oiio,mcanthony/oiio,YangYangTL/oiio,sambler/oiio,YangYangTL/oiio,sambler/oiio,lgritz/oiio,OpenImageIO/oiio,micler/oiio,cwilling/oiio,lgritz/oiio,micler/oiio,bdeluca/oiio,cwilling/oiio,OpenImageIO/oiio,lgritz/oiio,scott-wilson/oiio,scott-wilson/oiio,YangYangTL/oiio,YangYangTL/oiio,cwilling/oiio,mcanthony/oiio,sambler/oiio,mcanthony/oiio,scott-wilson/oiio,bdeluca/oiio,mcanthony/oiio,micler/oiio