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>

crates/egui_demo_app/src/apps/custom3d_glow.rs

@@ -1,5 +1,8 @@
 use std::sync::Arc;
 
+#[cfg(target_arch = "wasm32")]
+use core::any::Any;
+
 use eframe::egui_glow;
 use egui::mutex::Mutex;
 use egui_glow::glow;
@@ -48,6 +51,11 @@ impl eframe::App for Custom3d {
             self.rotating_triangle.lock().destroy(gl);
         }
     }
+
+    #[cfg(target_arch = "wasm32")]
+    fn as_any_mut(&mut self) -> &mut dyn Any {
+        &mut *self
+    }
 }
 
 impl Custom3d {