use a copy of `start` iterator when extracting bounding boxes + increase verbose output on failures
use a copy of `start` iterator when extracting bounding boxes + increase verbose output on failures

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