Demonstrate adding shader effects easily.

demos/calc.cpp

@@ -27,6 +27,7 @@ struct Calculator {
     for(auto& [name, cell] : $gui.cells()) {
       auto id = $gui.entity(name);
       $gui.set<guecs::Rectangle>(id, {});
+      $gui.set<guecs::Effect>(id, {});
       $gui.set<guecs::Label>(id, {guecs::to_wstring(name)});
       $gui.set<guecs::Clickable>(id, {
           [=](auto, auto) { fmt::println("clicked {}", name); }