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/sphere/sphere.js

@@ -12,7 +12,6 @@ goog.provide('ol.Sphere');
 
 goog.require('goog.array');
 goog.require('goog.math');
-goog.require('ol.geom.Polygon');
 
 
 
@@ -30,33 +29,6 @@ ol.Sphere = function(radius) {
 };
 
 
-/**
- * Returns an approximation to a circle centered on `center` with radius
- * `radius` with `n` distinct points.
- *
- * @param {ol.Coordinate} center Center.
- * @param {number} radius Radius.
- * @param {number=} opt_n N.
- * @return {ol.geom.Geometry} Circle geometry.
- * @todo api
- */
-ol.Sphere.prototype.circle = function(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, this.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;
-};
-
-
 /**
  * Returns the distance from c1 to c2 using the spherical law of cosines.
  *