Add an option to drop more points in dense areas than in sparse areas (#35)

* Add an option to drop the densest features by rate, not a fraction

* Restrict dot dropping just to dots, as it should be

* 50% mix of sparse and uniform point preservation

* Update version, changelog, test, and documentation

* 60% sparse, 40% dense

* Didn't update the test after the last change

* Make the drop-denser rate configurable without recompiling

geometry.cpp

@@ -1609,7 +1609,6 @@ drawvec polygon_to_anchor(const drawvec &geom) {
 	return drawvec();
 }
 
-
 static double dist(long long x1, long long y1, long long x2, long long y2) {
 	double dx = x2 - x1;
 	double dy = y2 - y1;