Limit guessed maxzoom to avoid spending too many tiles on polygon fill (#23)

* Change sqlite3 schema to deduplicate identical tiles

* Limit guessed maxzoom to avoid spending too many tiles on polygon fill

* Fix test.

These dust polygons now have their area calculated because their
maxzoom is being guessed, so the attributes from the largest one
rather than the last one are preserved.

* Increase polygon limit to a million tiles

* Two million tiles ought to be enough for anyone, right?

* Add explanatory comments for mysterious numbers

serial.hpp

@@ -150,6 +150,7 @@ struct serialization_state {
 
 	double *dist_sum = NULL;  // running tally for calculation of resolution within features
 	size_t *dist_count = NULL;
+	double *area_sum = NULL;
 	bool want_dist = false;
 
 	int maxzoom = 0;