use the line optimization for parallel segments

Use the line optimization when computing the bisector of two
parallel segments.

This also fixes a bug for inputs of the following form:

s 100 20 0 20
s 0 80 100 80
s 30 40 30 60

Signed-off-by: Panagiotis Cheilaris <philaris@cs.ntua.gr>

Segment_Delaunay_graph_Linf_2/include/CGAL/Segment_Delaunay_graph_Linf_2/Bisector_Linf.h

@@ -384,6 +384,8 @@ private:
 
     bool have_common_endp = is_mid_psrc or is_mid_ptrg;
 
+    bool optimize_for_line(false);
+
     Are_parallel_2 are_parallel;
 
     // compute supporting lines of segments
@@ -450,6 +452,8 @@ private:
       dinc = -d;
       dout = d;
 
+      optimize_for_line = true;
+
     } else {
       // here p and q are not parallel
 
@@ -629,6 +633,10 @@ private:
 
     Polychainline pcl (dinc, points, points + 1, dout);
 
+    if (optimize_for_line) {
+      pcl.set_line_optimization();
+    }
+
     return pcl;
 
   }