back to shared_ptr - reverts f8e60ab4372
back to shared_ptr - reverts f8e60ab4372

tomhughes/mapnik-vector-tile,tomhughes/mapnik-vector-tile,mapbox/mapnik-vector-tile,tomhughes/mapnik-vector-tile,mapbox/mapnik-vector-tile,mapbox/mapnik-vector-tile,tomhughes/mapnik-vector-tile,mapbox/mapnik-vector-tile