Move mbtiles code to its own source file

mbtiles.h

@@ -0,0 +1,7 @@
+sqlite3 *mbtiles_open(char *dbname, char **argv);
+
+void mbtiles_write_tile(sqlite3 *outdb, int z, int tx, int ty, const char *data, int size);
+
+void mbtiles_write_metadata(sqlite3 *outdb, char *fname, char *layername, int minzoom, int maxzoom, double minlat, double minlon, double maxlat, double maxlon, double midlat, double midlon, struct pool *fields);
+
+void mbtiles_close(sqlite3 *outdb, char **argv);