* Add support for a z_order field and sort output objects by z_order
PostGIS import tools like Osm2pgsql and Imposm can write a z_order field
to the database table in order to allow map styles to render features
ordered by road class and vertical layer. Tilemaker now gets a Lua
callback to set the z_order for an OSM object (default 0) and will sort
the vectortile features by z_order. z_order is also taken into account
for combining of features. z_order values are limited to 1-byte unsigned
integer.
* Document new ZOrder callback function
* Drop unnecessary variable
* Implement z_order sorting for the OpenMapTiles example config
This commit also adapts the minimum zoom levels to the lastest version.
* Parallel reading
* Fix win build
* Simplifier correctly init nodes
* Use generator to open input file(s)
* Bit more consistent naming of osm_store files
* Make number of locks in attribute store function of number of threads
* Small optimization, don't use virtual method calls
* Restore nodeListPolygon correct
* Don't drop self intersecting polygons
Richard Fairhurst