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

text.hpp

@@ -14,5 +14,6 @@ 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);
+std::string truncate_string(std::string const &s, size_t len);
 
 #endif