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.

serial.cpp

@@ -744,6 +744,7 @@ int serialize_feature(struct serialization_state *sst, serial_feature &sf, std::
 	    additional[A_GENERATE_POLYGON_LABEL_POINTS] ||
 	    sst->uses_gamma ||
 	    retain_points_multiplier > 1 ||
+	    preserve_multiplier_density_threshold > 0 ||
 	    cluster_distance != 0) {
 		sf.index = bbox_index;
 	} else {