Track the desired tile size for each zoom level in tileset metadata (#7)

tile.hpp

@@ -41,13 +41,15 @@ struct strategy {
 	size_t dropped_as_needed = 0;
 	size_t coalesced_as_needed = 0;
 	size_t detail_reduced = 0;
+	size_t tile_size = 0;
 
-	strategy(const atomic_strategy &s) {
+	strategy(const atomic_strategy &s, size_t ts) {
 		dropped_by_rate = s.dropped_by_rate;
 		dropped_by_gamma = s.dropped_by_gamma;
 		dropped_as_needed = s.dropped_as_needed;
 		coalesced_as_needed = s.coalesced_as_needed;
 		detail_reduced = s.detail_reduced;
+		tile_size = ts;
 	}
 
 	strategy() = default;