[Tile-serving] [osm2pgsql] Add area reprojection facility (#454)

[Paul Norman]

> + * \return the area in projected units, or in EPSG 3857 if area reprojection is enabled
> + */
> +double geometry_builder::getArea(const geos::geom::Geometry *geom) const
> +{
> +    // reprojection is not necessary, or has not been asked for.
> +    if (!reprojection) {
> +        return geom->getArea();
> +    }
> +
> +    // MultiPolygon - return sum of individual areas
> +    if (const geos::geom::MultiPolygon* multi = dynamic_cast<const geos::geom::MultiPolygon *>(geom)) {
> +        double area = 0.0;
> +        for (std::size_t i=0; i<multi->getNumGeometries(); i++) {
> +            area += getArea(multi->getGeometryN(i));
> +        }
> +        return area;

@lonvia we don't currently have any usage of `std::accumulate` or lambdas, but do you think this should be 
```c++
return std::accumulate(multi->begin(), multi->end(), 0.0,
                       [=](double a, const geos::geom::Geometry *geom) { return a + getArea(geom); });
```

---
Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/osm2pgsql/pull/454/files#r41619111
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