make constructor explicit to avoid C2664 error with VS2017.
make constructor explicit to avoid C2664 error with VS2017.

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