public static Viewport intersect(final Iterable<Viewport> vps) {
        return intersect(vps, vp -> vp);
    }
internal worker function for
- containing(Points)
- containingGCLiveCaches(Geocaches)
- containingCachesAndWaypoints(Geocaches)
main/src/main/java/cgeo/geocaching/location/Viewport.java