Adding and removing points (segments) to the Beziergon did not work as
before due to changes from 62dab7da. Actually that commit did not take
into account, that bezierconn_closest_segment() returned 0 for the first
segment but beziershape_closest_segment() returned 1.
Now the segment index is zero-based as expected and the Beziergon code
use the appropriate offsets to deal with it.
Also a unit-test is specifically written for Add/Remove Segment to not
introduce such bugs with upcoming restructuring.