examples/winit: Implement proper Event::Resumed semantics

On Android the backing buffer (`NativeWindow`) disappears when the
application is not focussed and/or the screen is locked.  Winit handles
this by requiring apps to create their `raw_window_handle()` consumers
_after_ `Event::Resumed` and to clean it up _before_ returning from
`Event::Suspended`.  For consistency Winit also sends `Resumed` on all
other platforms during init.

examples/winit.rs

@@ -23,18 +23,29 @@ fn main() {
             .unwrap();
     }
 
-    let context = softbuffer::Context::new(window.clone()).unwrap();
-    let mut surface = softbuffer::Surface::new(&context, window.clone()).unwrap();
+    let mut state = None;
 
     event_loop
         .run(move |event, elwt| {
             elwt.set_control_flow(ControlFlow::Wait);
 
             match event {
+                Event::Resumed => {
+                    let context = softbuffer::Context::new(window.clone()).unwrap();
+                    let surface = softbuffer::Surface::new(&context, window.clone()).unwrap();
+                    state = Some((context, surface));
+                }
+                Event::Suspended => {
+                    state = None;
+                }
                 Event::WindowEvent {
                     window_id,
                     event: WindowEvent::RedrawRequested,
                 } if window_id == window.id() => {
+                    let Some((_, surface)) = state.as_mut() else {
+                        eprintln!("RedrawRequested fired before Resumed or after Suspended");
+                        return;
+                    };
                     if let (Some(width), Some(height)) = {
                         let size = window.inner_size();
                         (NonZeroU32::new(size.width), NonZeroU32::new(size.height))