Choose the megatile features from those that will be in the next N zooms (#280)

* Choose the megatile features from those that will be in the next N zooms

* Take fractional zooms into account in multiplier feature choices

* Fix more tests

* Add a flag to retain multiplier features by minimum distance

* Limit feature expansion from multiplier density to 2x

* The multiplier cap was a bad idea

* Revert "The multiplier cap was a bad idea"

This reverts commit 6f8273a4c8.

* Revert "Limit feature expansion from multiplier density to 2x"

This reverts commit a26e41309d.

* Revert "Add a flag to retain multiplier features by minimum distance"

This reverts commit 01f14a4255.

* Remove the multiplier sequence, which should no longer matter

* Revert "Revert "Add a flag to retain multiplier features by minimum distance""

This reverts commit 776da4a1b8.

* Revert "Revert "Limit feature expansion from multiplier density to 2x""

This reverts commit 44a683d808.

* Revert "Revert "The multiplier cap was a bad idea""

This reverts commit 80f7cb1c0e.

* Track two kinds of previous index for next_feature

* Fix multiplier density threshold, I think

* Oh, I didn't git add the code changes

* Update version and changelog

* Try to install sqlite3 to fix the automated build

* Deleted too much

* Only let --preserve-point-density-threshold shift density around

* Remove the density debt concept, since it doesn't help

* Make the drop states a vector instead of an array

* Revert "Make the drop states a vector instead of an array"

This reverts commit 66c7abb6fa.

* Revert "Remove the density debt concept, since it doesn't help"

This reverts commit 707bb0c562.

* Revert "Only let --preserve-point-density-threshold shift density around"

This reverts commit ecf01f2231.

main.hpp

@@ -65,6 +65,7 @@ extern long long extend_zooms_max;
 extern int retain_points_multiplier;
 extern size_t maximum_string_attribute_length;
 extern std::string accumulate_numeric;
+extern unsigned long long preserve_multiplier_density_threshold;
 
 struct order_field {
 	std::string name;