reorganise vipspng a little
reorganise vipspng a little

so we only set max malloc in one place

see https://github.com/libvips/libvips/pull/1424

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