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 
> A name avoiding the 'circle' word should be preferable to avoid all
possible confusion between a circle, which is a mathematical object and a
`circular` polygon, which is a polygon that is created looking like a
circle.

Actually that function creates a regular polygon. So 'createRegular' would
be an appropriate name choice.