add ack for dzsave tile skip
add ack for dzsave tile skip

see https://github.com/jcupitt/libvips/issues/352

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