Improve the appearance of coalesce-densest-as-needed tiles (#247)

* Start to distinguish fixed cluster density setting from as-needed density

* Make consistent {drop,coalesce}-densest decisions between zooms

* Actually track the previous index instead of just intending to

* Clean up collinearities in coalesced features

* To determine densest, look at actual physical distance, not just index

* Don't actually need the previous index in serial_feature now

* Center of mass of one feature to most distant point of the next

* Add apologetic comment

* Wait, how did the tests pass before?

* Revert "Wait, how did the tests pass before?"

This reverts commit f73c8ee543.

* Add --maximum-string-attribute-length option

* Update version and changelog

* A little more testing to make sure

geometry.hpp

@@ -114,4 +114,6 @@ std::string overzoom(const std::string &s, int oz, int ox, int oy, int nz, int n
 		     std::unordered_map<std::string, attribute_op> const &attribute_accum,
 		     std::vector<std::string> const &unidecode_data);
 
+draw center_of_mass_mp(const drawvec &dv);
+
 #endif