Relation#multipolygon returns coordinate arrays
Relation#asGeoJSON
Better Way#asGeoJSON
Improve multipolygon rendering

Multipolygon relations report their geometry as 'area' and are rendered
as such. However, they do not render a stroke. The stroke rendering
will come from the individual lines, which are given the tag
classes of their parent relations, allowing them to have a stroke
style matching the style of simple areas with the same tags.

Untagged circular ways are no longer considered areas. This prevents
an untagged inner way of a multipolygon from rendering as an area and
is consistent with how P2 and JOSM treat them.

In the CSS, it's no longer necessary to deal with multipolygons
explicitly in selectors. But keep in mind that area boundaries can
now be rendered either as lines or as area strokes. In most cases
the selector should be `path.stroke.tag-_____`, i.e. an explicit
`.area` or `.line` classes should not be included.

Finally, the parent ways of selected multipolygons are given the 'selected'
class.