add random shifts to find_intersection()

closes libMesh#3496

Author: miaoyinb

src/mesh/mesh_triangle_holes.C

@@ -105,11 +105,14 @@ namespace
   //
   // If the intersection is a "glancing" one at a corner, return -1.
   Real find_intersection(const Point & source,
-                         const Point & ray_target,
+                         const Point & ray_target_0,
                          const Point & edge_pt0,
                          const Point & edge_pt1,
                          const Point & edge_pt2)
   {
+    // Add a random small shift to the source and three points
+    const Point ray_target = ray_target_0 + Point((Real)(rand() % 5000 + 5000) / 5000.0 * libMesh::TOLERANCE * libMesh::TOLERANCE, (Real)(rand() % 5000 + 5000) / 5000.0 * libMesh::TOLERANCE * libMesh::TOLERANCE, 0.0);
+
     // Quick and more numerically stable check
     if (!is_intersection(source, ray_target, edge_pt0, edge_pt1))
       return -1;