eframe web: access app from WebHandle (#1886)

Co-authored-by: Stanislav <enomado@users.noreply.github.com>
Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

examples/custom_3d_three-d/src/main.rs

@@ -70,6 +70,11 @@ impl eframe::App for MyApp {
             });
         });
     }
+
+    #[cfg(target_arch = "wasm32")]
+    fn as_any_mut(&mut self) -> &mut dyn Any {
+        &mut *self
+    }
 }
 
 /// We get a [`glow::Context`] from `eframe` and we want to construct a [`ThreeDApp`].