[Tile-serving] [osm2pgsql] Allow polygon/linestring decision to be set by value in lua (#346)

[math1985]

This allows the decision whether an object is treated as polygon or linestring
to be set in the style.lua file.

In particular, it treats man_made=embankment and natural=cliff as linestring,
and highway=services as polygon.

Including this in the default style.lua file has the following advantages:
* It serves as an easy-to-extend example of how the polygon/linestring decision
  can be made based on key.
* It provides sane defaults - it can be expected that most users of the style
  want to treat man_made=embankment and natural=cliff as linestring, and
  highway=services as polygon.

This resolves the following downstream bugs in openstreetmap-carto:
* https://github.com/gravitystorm/openstreetmap-carto/issues/892
* https://github.com/gravitystorm/openstreetmap-carto/issues/268
* https://github.com/gravitystorm/openstreetmap-carto/issues/137

This builds on #345, and thus assumes #345 is merged first.
You can view, comment on, or merge this pull request online at:

  https://github.com/openstreetmap/osm2pgsql/pull/346

-- Commit Summary --

  * Add documentation and improve legibility style.lua
  * Allow polygon/linestring decision to be set by value in lua

-- File Changes --

    M style.lua (176)

-- Patch Links --

https://github.com/openstreetmap/osm2pgsql/pull/346.patch
https://github.com/openstreetmap/osm2pgsql/pull/346.diff

---
Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/osm2pgsql/pull/346
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