simplify impl + const correctness + minor cleanup
simplify impl + const correctness + minor cleanup

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