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.

Fix #2680 
Right I was also thinking about RegularPolygon as we had in OpenLayers 2:

https://github.com/openlayers/openlayers/blob/master/lib/OpenLayers/Handler/RegularPolygon.js