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

@@ -22,18 +22,29 @@ 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))