/*******************************************************/
/* adds the possible crossings between segments [A,B] (with end-point global indices i and i_next) */
/*and segments [C,D] and [E,F] to the list inter and updates _End_segments */
/*******************************************************/
/* adds the possible crossings between segments [A,B] (with end-point global indices i and i_next) */
/*and segments [C,D] and [E,F] to the list inter and updates _End_segments */