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

text.hpp

@@ -11,5 +11,6 @@ std::string format_commandline(int argc, char **argv);
 unsigned long long fnv1a(std::string const &s);
 unsigned long long fnv1a(const char *s, char additional);
 unsigned long long fnv1a(size_t size, void *p);
+unsigned long long bit_reverse(unsigned long long v);
 
 #endif