Don't preflight zoom levels for potential as-needed dropping (#40)

* Working on eliminating preflighting

* Adjust for the map/images schema change

* Avoid generating duplicate tiles with the detail reduction strategy

* Do error checking if tiles in a directory can't be written

* Update changelog and version

* Revert unintentional code reordering

* Neglected to add the exit on error here

dirtiles.hpp

@@ -6,6 +6,7 @@
 #define DIRTILES_HPP
 
 void dir_write_tile(const char *outdir, int z, int tx, int ty, std::string const &pbf);
+void dir_erase_zoom(const char *outdir, int z);
 
 void check_dir(const char *d, char **argv, bool force, bool forcetable);