Fix incorrect mirror formula
Fix incorrect mirror formula

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