Avoid placing polygon labels in holes (#62)

* Avoid placing polygon labels in holes

* Traverse vertices in hilbert order to find potential label points

* Update test to reflect new label placements

* Sorting by Y coordinate is better and easier than Hilbert order

* Give a bonus for being near the center of mass

* Make label points a little more border-shy

* Tall places, not just wide places; border lines, not just border points

* Clean up

* Try diagonals through the features too

* Limit the search for label points to prevent slowing down too much

* At this point the center of mass bonus is doing more harm than good

* Forgot to update test

CHANGELOG.md

@@ -1,3 +1,7 @@
+## 2.21.0
+
+* Improve label placement to avoid placing labels in polygon holes
+
 ## 2.20.0
 
 * Round coordinates instead of truncating them, for better precision when overzooming