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

plugin.cpp

@@ -428,7 +428,9 @@ serial_feature parse_feature(json_pull *jp, int z, unsigned x, unsigned y, std::
 
 				json_object *dropped = json_hash_get(tippecanoe, "dropped");
 				if (dropped != NULL && dropped->type == JSON_TRUE) {
-					sf.dropped = true;
+					sf.dropped = FEATURE_DROPPED;  // dropped
+				} else {
+					sf.dropped = FEATURE_KEPT;  // kept
 				}
 			}