Trying to follow Vlad's suggestion and work from the top down.

Doesn't quite work yet, but seems like the right thing to do.
From the bottom up, indexing is a mess because of the buffering,
and includes unneeded areas in the bounding boxes of big features.

enumerate.c

@@ -24,7 +24,7 @@ void enumerate(char *fname) {
 		long long x = sqlite3_column_int(stmt, 1);
 		long long y = sqlite3_column_int(stmt, 2);
 
-		y = (1LL << zoom) - y;
+		y = (1LL << zoom) - 1 - y;
 		printf("%s %lld %lld %lld\n", fname, zoom, x, y);
 	}