Consistent circle transform

Depending on compilation flags, applying a transform used to either:
- work;
- fail throwing an abstract method not implemented message;
- fail silently.

Now it should consistently work, like the other geometries.

Adding a polygon factory method creating an approximation of a circle on
a plane would be useful for users wanting the circle to be deformed.
It would be similar to the `circular` function which creates an
approximation of a circle on a sphere.

src/ol/geom/geometry.js

@@ -173,7 +173,8 @@ ol.geom.Geometry.prototype.getType = goog.abstractMethod;
 
 
 /**
- * Apply a transform function to the geometry.  Modifies the geometry in place.
+ * Apply a transform function to each coordinate of the geometry.
+ * The geometry is modified in place.
  * If you do not want the geometry modified in place, first clone() it and
  * then use this function on the clone.
  * @function
@@ -184,8 +185,9 @@ ol.geom.Geometry.prototype.applyTransform = goog.abstractMethod;
 
 
 /**
- * Transform a geometry from one coordinate reference system to another.
- * Modifies the geometry in place.
+ * Transform each coordinate of the geometry from one coordinate reference
+ * system to another. The geometry is modified in place.
+ * For example, a line will be transformed to a line and a circle to a circle.
  * If you do not want the geometry modified in place, first clone() it and
  * then use this function on the clone.
  *