Drop or retain whole multiplier clusters when dropping as needed (#198)

* Prep to track conditions other than just "dropped" or "kept"

* Count up instead of down

* Drop or retain whole multiplier clusters based on their first feature

* Calculate a global feature dropping sequence

* Switch over to using the drop sequence for drop-fraction

* Remove unused arguments for the old drop-fraction implementation

* Fix copy-and-paste bugs, update tests

* Properly incorporate feature_minzoom into the drop sequence, I hope

* Rename drop_by to drop_sequence

* See if sorting within clusters fixes filter stability between zooms

* Remove very chatty debug print

* Update changelog and version

* Use named constants instead of numbers for feature dropping/keeping

* Add comment to explain purpose and method of bit reversal

mvt.hpp

@@ -54,7 +54,7 @@ struct mvt_feature {
 	int /* mvt_geometry_type */ type = 0;
 	unsigned long long id = 0;
 	bool has_id = false;
-	bool dropped = false;
+	int dropped = 0;
 	size_t seq = 0;	 // used for ordering in overzoom
 
 	mvt_feature() {