Fix sometimes pruning entire branches that are valid

An example can be see in this issue: https://github.com/CGAL/cgal/issues/3993.
This configuration:
- we leave from a vertex v0 and want to reach v1
- a (useful) cone gets split by a vertex v2
- v3 is such that v0v2 = v2v3 and v1 is on the left of the line
- we first reach v3 from the right
- when we reach v3 from the left, with a cone that also contains v1, this cone
  is discarded because we have already reached v3 from the right and the time
  is not better (it's equal). Thus we can't reach v1 with the direct cone
  from v1.

Other artificial configurations can be constructed noting that the case
of identical source-ray target for both nodes where not really handled.

Surface_mesh_shortest_path/include/CGAL/Surface_mesh_shortest_path/Surface_mesh_shortest_path.h

@@ -42,6 +42,7 @@
 #include <CGAL/assertions.h>
 #include <CGAL/AABB_tree.h>
 #include <CGAL/Default.h>
+#include <CGAL/enum.h>
 #include <CGAL/number_utils.h>
 
 #include <CGAL/Surface_mesh_shortest_path/barycentric.h>
@@ -1048,8 +1049,6 @@ private:
   */
   void process_node(Cone_tree_node* node)
   {
-    typename Traits::Compare_relative_intersection_along_segment_2 crias2(m_traits.compare_relative_intersection_along_segment_2_object());
-
     bool leftSide = false;
     bool rightSide = false;
 
@@ -1106,44 +1105,30 @@ private:
       Node_distance_pair currentOccupier = m_vertexOccupiers[entryEdgeIndex];
       FT currentNodeDistance = node->distance_from_target_to_root();
 
-      bool isLeftOfCurrent = false;
-
       if (m_debugOutput)
       {
         std::cout << "\t Entry Edge = " << entryEdgeIndex << std::endl;
         std::cout << "\t Target vertex = " << get(m_vertexIndexMap, node->target_vertex()) << std::endl;
       }
 
+      // the relative position of the ray between node.source() and node.target_vertex() and the ray
+      // from occupier.source() (-1 left, 0 collinear, 1 right)
+      CGAL::Sign s;
+
       if (currentOccupier.first != NULL)
       {
-        if (node->is_vertex_node())
-        {
-          isLeftOfCurrent = false;
-        }
-        else if (currentOccupier.first->is_vertex_node())
-        {
-          isLeftOfCurrent = true;
-        }
-        else
-        {
-          CGAL::Comparison_result comparison = crias2(
-            node->entry_segment(),
-            node->ray_to_target_vertex().supporting_line(),
-            currentOccupier.first->entry_segment(),
-            currentOccupier.first->ray_to_target_vertex().supporting_line()
-          );
+        CGAL_assertion(m_graph.is_valid());
+        CGAL_assertion(node->entry_edge() == currentOccupier.first->entry_edge());
+        CGAL_assertion(node->target_vertex() == currentOccupier.first->target_vertex());
 
-          if (comparison == CGAL::SMALLER)
-          {
-            isLeftOfCurrent = true;
-          }
-        }
+        typename Traits::Orientation_2 o2(m_traits.orientation_2_object());
+        s = o2(currentOccupier.first->source_image(), currentOccupier.first->target_point(), node->source_image());
 
         if (m_debugOutput)
         {
           std::cout << "\t Current occupier = " << currentOccupier.first << std::endl;
           std::cout << "\t Current Occupier Distance = " << currentOccupier.second << std::endl;
-          std::cout << "\t " << (isLeftOfCurrent ? "Left" : "Right") << " of current" << std::endl;
+          std::cout << "\t left/collinear/right turn? " << s << std::endl;
         }
       }
 
@@ -1181,7 +1166,7 @@ private:
 
         if (currentOccupier.first != NULL)
         {
-          if (isLeftOfCurrent)
+          if (s != RIGHT_TURN)
           {
             if (currentOccupier.first->get_left_child())
             {
@@ -1193,7 +1178,8 @@ private:
               currentOccupier.first->m_pendingLeftSubtree = NULL;
             }
           }
-          else
+
+          if(s != LEFT_TURN)
           {
             if (currentOccupier.first->get_right_child())
             {
@@ -1259,14 +1245,12 @@ private:
       }
       else // there is already an occupier, at a closer distance
       {
-        if (isLeftOfCurrent)
-        {
+        if (s != RIGHT_TURN)
           propagateLeft = true;
-        }
-        else if (!node->is_source_node())
-        {
+        if (s != LEFT_TURN)
+          propagateRight = true;
+        if (!node->is_source_node())
           propagateRight = true;
-        }
       }
     }
     else // interval node that does not contain the target vertex