Add an option to generate label points in place of polygons (#20)

* Add an option to generate label points in place of polygons

* Change all these places where I said "extent" but really meant "area"

* Revert "Change all these places where I said "extent" but really meant "area""

This reverts commit 403828d2f7.

* Add --order-smallest-first and --order-largest-first options

* Use Turf's center-of-mass algorithm for polygon label points

* If the label point isn't within the polygon, find one that is

* Don't choose a label point that is too close to a border

* Try a little harder to find an optimal label point

* Checkerboard which tiles labels are generated in, to reduce adjacency

* Use a label point for the general representative point for polygons

(Skipping the iteration to find one that is as far as possible from
the borders)

This makes the labels look better in many cases (like France at z1)
but unfortunately ripples into changing the sequence of polygons in
many tests, so the diff is big.

* Revert "Use a label point for the general representative point for polygons"

This reverts commit 2261adf05e.

* Checkpoint work on spiral labels

* Clip label spirals to the feature bounds

* Fix label test

* Be careful not to place spiral labels too close to borders either

* For spiral anchors, only check tile scale, not feature size

* Update test

* Only try to find a central label point for the largest ring

* In tiny polygon dust, keep the attributes of the largest feature

main.hpp

@@ -66,6 +66,10 @@ struct order_field {
 
 extern std::vector<order_field> order_by;
 
+// not legal UTF-8, so can't appear as a real attribute name
+#define ORDER_BY_SIZE "\200size"
+extern bool order_by_size;
+
 int mkstemp_cloexec(char *name);
 FILE *fopen_oflag(const char *name, const char *mode, int oflag);
 bool progress_time();