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

serial.hpp

@@ -89,9 +89,16 @@ struct serial_feature {
 	// to be provided by frontends:
 
 	long long bbox[4] = {0, 0, 0, 0};
-	bool dropped = false;  // was this feature dropped by rate?
-	drawvec edge_nodes;    // what nodes at the tile edge were added during clipping?
+	drawvec edge_nodes;  // what nodes at the tile edge were added during clipping?
 
+#define FEATURE_DROPPED -1
+#define FEATURE_KEPT 0
+	// <0: dropped
+	//  0: kept
+	// >0: sequence number of additional feature kept by retain-points-multiplier
+	int dropped = FEATURE_DROPPED;	// was this feature dropped by rate?
+
+	// unsigned long long drop_by;  // dot-dropping priority
 	bool reduced;	   // is polygon dust
 	bool coalesced;	   // was coalesced from multiple features
 	int line_detail;   // current tile resolution being used for simplification