Support type=boundary relations as native multipolygons

[systemed]

This enables boundary relations to benefit from multipolygon inner/outer processing.

You can now treat boundary relations in one of two ways:

• as properties on a way, iterated through with NextRelation (this is good for administrative boundaries which are often coterminous)
• as discrete geometries, processed with relation_function (this is good for nature reserves, forests and other polygons)

An example of the latter would be:

function relation_scan_function(relation)
  if relation:Find("type")=="boundary" then
    relation:Accept()
  end
end

function relation_function(relation)
  if relation:Find("type")=="boundary" and relation:Find("boundary")=="protected_area" then
    relation:Layer("park",true)
    relation:Attribute("class", "public_park")
    SetNameAttributes(relation)
  end
end

In essence, any relation with inner/outer roles will be processed as a multipolygon by relation_function, apart from type=multipolygon which is processed by way_function. To process relations other than type=multipolygon, writing a relation_scan_function is still required.