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 
> The way we formutate this we really give the impression that Circle is special and obey special rules.

Which is true. If you reproject every point of a point, line or polygon the transformed geometry is still a point, line, polygon, respectively. If you reproject every point of a polygon whose shape is "circular" the shape of the transformed may not be circular.