Point-in-polygon algorithm

No tests. I didn't write it so I assume it's correct!

js/id/util.js

@@ -107,3 +107,29 @@ iD.util.geo.chooseIndex = function(way, point, map) {
         loc: loc
     };
 };
+
+// Return whether point is contained in polygon.
+//
+// `point` should be a 2-item array of coordinates.
+// `polygon` should be an array of 2-item arrays of coordinates.
+//
+// From https://github.com/substack/point-in-polygon.
+// ray-casting algorithm based on
+// http://www.ecse.rpi.edu/Homepages/wrf/Research/Short_Notes/pnpoly.html
+//
+iD.util.geo.pointInPolygon = function(point, polygon) {
+    var x = point[0],
+        y = point[1],
+        inside = false;
+
+    for (var i = 0, j = polygon.length - 1; i < polygon.length; j = i++) {
+        var xi = polygon[i][0], yi = polygon[i][1];
+        var xj = polygon[j][0], yj = polygon[j][1];
+
+        var intersect = ((yi > y) != (yj > y))
+            && (x < (xj - xi) * (y - yi) / (yj - yi) + xi);
+        if (intersect) inside = !inside;
+    }
+
+    return inside;
+};