Change cast to derived type to .derived() call

Former-commit-id: 08cba2bf250743d185ce99e87f4c9a04913eb793

include/igl/vertex_triangle_adjacency.cpp

@@ -70,7 +70,7 @@ IGL_INLINE void igl::vertex_triangle_adjacency(
   // vfd now acts as a counter
   vfd = NI;
 
-  static_cast<DerivedVF &>(VF )= Eigen::VectorXi(3*F.rows());
+  VF.derived()= Eigen::VectorXi(3*F.rows());
   for (int i = 0; i < F.rows(); i++)
   {
     for (int j = 0; j < 3; j++)