Add missing config impl structs

viewer/glow-extras/viewer/detail/config_structs_impl.hh

@@ -2,4 +2,13 @@
 
 namespace glow::viewer::detail {
 
+struct no_grid_impl
+{
+};
+struct no_shadow_impl
+{
+};
+struct print_mode_impl
+{
+};
 }