The cairo library does not handle arcs natively. Actually the `CPML` library is able to transparently convert them to Bézier curves when needed.

Add more logic to do the same in the reverse side, that is to automatically convert curves to arcs when a curve is close enough to an arc approximation. This will bring an additional information that can be used in some operation, such as getting the value to put in the quote of a radial dimension or offsetting a segment in a precise way (offsetting cubic Bézier curves usually gives an approximation, see issue T10 for details).