Box the app creator (#1373)

eframe/examples/svg.rs

@@ -11,7 +11,11 @@ fn main() {
         initial_window_size: Some(egui::vec2(1000.0, 700.0)),
         ..Default::default()
     };
-    eframe::run_native("svg example", options, |_cc| Box::new(MyApp::default()));
+    eframe::run_native(
+        "svg example",
+        options,
+        Box::new(|_cc| Box::new(MyApp::default())),
+    );
 }
 
 struct MyApp {