minimise shared_ptr assignments and simplify code
minimise shared_ptr assignments and simplify code

naturalatlas/mapnik,mapnik/mapnik,naturalatlas/mapnik,tomhughes/mapnik,tomhughes/mapnik,naturalatlas/mapnik,tomhughes/mapnik,mapnik/mapnik,lightmare/mapnik,mapnik/mapnik,lightmare/mapnik,lightmare/mapnik,naturalatlas/mapnik,lightmare/mapnik,tomhughes/mapnik,mapnik/mapnik