fix anim webp delay check
fix anim webp delay check

only test frame duration for animated webp images

thanks Kleis

see https://github.com/libvips/libvips/pull/1257

jcupitt/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips,lovell/libvips,lovell/libvips,jcupitt/libvips