see: https://github.com/jcupitt/libvips/issues/160 we were using UNBUFFERED, but on very-many-core systems this can get out of order enough to make vipspng free the read object early
see: https://github.com/jcupitt/libvips/issues/160 we were using UNBUFFERED, but on very-many-core systems this can get out of order enough to make vipspng free the read object early