Not updating prev_x and prev_y in integral polyline for the compute_segments_normals and compute_segments_tangents functions

An error, which was reported to me, is the non-update the x_prev and y_prev variables in the loops of the compute_segments_normals and computation_segments_tangents functions. It should then be added after having calculated the normal or the tangent: prev_x, prev_y = x, y

Because otherwise the calculation of the normal or the tangent is not carried out for each segment