Point-in-polygon improvements

As suggested by @tschaub in #674, geom.pointInPoly is not needed
if we have geom.LinearRing#containsCoordinate. This pull request
also adds tests and documentation on the limitations of the
containsCoordinate method.

I think for now it is ok to keep geometry/topology functions as
simple as possible in ol3. If we decide to not rely on third
party libraries like jsts for topology operations, we can always
refine what we have and e.g. port topology operations over from
ol2.

src/ol/geom/linearring.js

@@ -33,3 +33,33 @@ goog.inherits(ol.geom.LinearRing, ol.geom.LineString);
 ol.geom.LinearRing.prototype.getType = function() {
   return ol.geom.GeometryType.LINEARRING;
 };
+
+
+/**
+ * Check whether a given coordinate is inside this ring. Note that this is a
+ * fast and simple check - points on an edge or vertex of the ring are either
+ * classified inside or outside.
+ *
+ * @param {ol.Coordinate} coordinate Coordinate.
+ * @return {boolean} Whether the coordinate is inside the ring.
+ */
+ol.geom.LinearRing.prototype.containsCoordinate = function(coordinate) {
+  // http://www.ecse.rpi.edu/Homepages/wrf/Research/Short_Notes/pnpoly.html
+  var x = coordinate[0], y = coordinate[1];
+  var vertices = this.getCoordinates();
+  var inside = false;
+  var xi, yi, xj, yj, intersect;
+  var numVertices = vertices.length;
+  for (var i = 0, j = numVertices - 1; i < numVertices; j = i++) {
+    xi = vertices[i][0];
+    yi = vertices[i][1];
+    xj = vertices[j][0];
+    yj = vertices[j][1];
+    intersect = ((yi > y) != (yj > y)) &&
+        (x < (xj - xi) * (y - yi) / (yj - yi) + xi);
+    if (intersect) {
+      inside = !inside;
+    }
+  }
+  return inside;
+};