fix warnings for Eigen's 'convenience' type all (#2421)

tutorial/406_FastAutomaticSkinningTransformations/main.cpp

@@ -2,6 +2,7 @@
 
 #include <igl/readDMAT.h>
 #include <igl/readOBJ.h>
+#include <igl/placeholders.h>
 #include <igl/arap.h>
 #include <igl/arap_dof.h>
 #include <igl/opengl/glfw/Viewer.h>
@@ -146,7 +147,7 @@ int main(int argc, char *argv[])
   // Plot the mesh with pseudocolors
   igl::opengl::glfw::Viewer viewer;
   viewer.data().set_mesh(U, F);
-  viewer.data().add_points(V(b,Eigen::placeholders::all),sea_green);
+  viewer.data().add_points(V(b,igl::placeholders::all),sea_green);
   viewer.data().show_lines = false;
   viewer.callback_pre_draw = &pre_draw;
   viewer.callback_key_down = &key_down;