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

CHANGELOG.md

@@ -1,3 +1,8 @@
+# 2.56.0
+
+* Rework --coalesce-densest-as-needed and --drop-densest-as-needed to look better
+* Add --maximum-string-attribute-length option
+
 # 2.55.0
 
 * Fix hash collisions in the string pool