Move ol.Sphere#circle to ol.geom.Polygon.circular

Previously, ol.geom.Polygon was a transitive dependency of ol.proj (since ol.proj requires ol.sphere.NORMAL, and all spheres were capable of generating circular polygons).  Instead, ol.proj should be lower-level.  Since it deals only with coordinate arrays, it shouldn't depend on all of the geometry code.

By adding a static `circular` function to `ol.geom.Polygon`, the dependency tree makes more sense.  If you want to create a polygon that approximates a circle on a sphere, you require `ol.geom.Polygon` and `ol.Sphere` (or one of the constants).

This makes room for geometries to have a `transform` method that takes projection-like arguments (meaning that `ol.geom.Geometry` will require `ol.proj`).

src/ol/geom/polygon.js

@@ -314,3 +314,29 @@ ol.geom.Polygon.prototype.setFlatCoordinates =
   this.ends_ = ends;
   this.dispatchChangeEvent();
 };
+
+
+/**
+ * Create an approximation of a circle on the surface of a sphere.
+ * @param {ol.Sphere} sphere The sphere.
+ * @param {ol.Coordinate} center Center.
+ * @param {number} radius Radius.
+ * @param {number=} opt_n Optional number of points.  Default is `32`.
+ * @return {ol.geom.Polygon} Circle geometry.
+ * @todo api
+ */
+ol.geom.Polygon.circular = function(sphere, center, radius, opt_n) {
+  var n = goog.isDef(opt_n) ? opt_n : 32;
+  /** @type {Array.<number>} */
+  var flatCoordinates = [];
+  var i;
+  for (i = 0; i < n; ++i) {
+    goog.array.extend(
+        flatCoordinates, sphere.offset(center, radius, 2 * Math.PI * i / n));
+  }
+  flatCoordinates.push(flatCoordinates[0], flatCoordinates[1]);
+  var polygon = new ol.geom.Polygon(null);
+  polygon.setFlatCoordinates(
+      ol.geom.GeometryLayout.XY, flatCoordinates, [flatCoordinates.length]);
+  return polygon;
+};