Rename every thing that i was implemented for multiples windows to Viewport

crates/egui_demo_app/src/backend_panel.rs

@@ -283,7 +283,7 @@ impl BackendPanel {
                             log::info!("Request a repaint in 3s...");
                             ctx.request_repaint_after(
                                 std::time::Duration::from_secs(3),
-                                ctx.current_rendering_window(),
+                                ctx.current_rendering_viewport(),
                             );
                         });
                     }