Fix missing return in ImageBugAlgo::flatten().
Fix missing return in ImageBugAlgo::flatten().

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