Allow polygon/linestring decision to be set by value in lua

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:
* gravitystorm/openstreetmap-carto#892
* gravitystorm/openstreetmap-carto#268
* gravitystorm/openstreetmap-carto#137

This builds on osm2pgsql-dev#345, and thus assumes osm2pgsql-dev#345 is merged first.

style.lua

@@ -6,6 +6,12 @@ polygon_keys = { 'building', 'landuse', 'amenity', 'harbour', 'historic', 'leisu
       'public_transport', 'shop', 'sport', 'tourism', 'waterway',
       'wetland', 'water', 'aeroway' }
 
+-- Objects with any of the following key/value combinations will be treated as polygon
+polygon_values = {highway='services'}
+
+-- Objects with any of the following key/value combinations will be treated as linestring
+linestring_values = {man_made='embankment', natural='cliff'}
+
 -- Objects without any of the following keys will be deleted
 generic_keys = {'access','addr:housename','addr:housenumber','addr:interpolation','admin_level','aerialway','aeroway','amenity','area','barrier',
    'bicycle','brand','bridge','boundary','building','capital','construction','covered','culvert','cutting','denomination','disused','ele',
@@ -126,6 +132,22 @@ function filter_tags_way (keyvalues, numberofkeys)
          break
       end
    end
+
+  -- Treat objects with a key/value combination in polygon_values as polygon
+   for k,v in pairs(polygon_values) do
+      if keyvalues[k] == v then
+         polygon=1
+         break
+      end
+   end
+
+  -- Treat objects with a key/value combination in linestring_values not as polygon
+   for k,v in pairs(linestring_values) do
+      if keyvalues[k] == v then
+         polygon=0
+         break
+      end
+   end
 
    -- Treat objects tagged as area=yes, area=1, or area=true as polygon,
    -- and treat objects tagged as area=no, area=0, or area=false not as polygon
@@ -168,14 +190,29 @@ function filter_tags_relation_member (keyvalues, keyvaluemembers, roles, memberc
       -- Treat as polygon
       polygon = 1
       polytagcount = 0;
-      -- Count the number of polygon tags of the object
+      -- Count the number of polygon tags
+      -- First count keys in polygon_keys
       for i,k in ipairs(polygon_keys) do
          if keyvalues[k] then
             polytagcount = polytagcount + 1
          end
       end
-      -- If there are no polygon tags, add tags from all outer elements to the multipolygon itself
-      if (polytagcount == 0) then
+      -- Then add key/value combinations in polygon_values
+      for k,v in pairs(polygon_values) do
+         if keyvalues[k] == v then
+            polytagcount = polytagcount + 1
+         end
+      end
+      -- Then substract key/value combinations in linestring_values
+      for k,v in pairs(linestring_values) do
+         if keyvalues[k] == v then
+            polytagcount = polytagcount - 1
+         end
+      end
+
+      -- If the multipolygon has no polygon keys or polygon key/value combinations,
+      -- add tags from all outer elements to the multipolygon itself
+      if (polytagcount <= 0) then
          for i = 1,membercount do
             if (roles[i] == "outer") then
                for k,v in pairs(keyvaluemembers[i]) do