Don't update when suspended to avoid GPU use on iOS. (#2482)

# Objective

This fixes a crash caused by iOS preventing GPU access when not focused: #2296

## Solution

This skips `app.update()` in `winit_runner` when `winit` sends the `Suspended` event, until `Resumed`.

I've tested that this works for me on my iOS app.

crates/bevy_winit/src/lib.rs

@@ -233,6 +233,8 @@ pub fn winit_runner_with(mut app: App, mut event_loop: EventLoop<()>) {
         .get_resource::<WinitConfig>()
         .map_or(false, |config| config.return_from_run);
 
+    let mut active = true;
+
     let event_handler = move |event: Event<()>,
                               event_loop: &EventLoopWindowTarget<()>,
                               control_flow: &mut ControlFlow| {
@@ -475,14 +477,22 @@ pub fn winit_runner_with(mut app: App, mut event_loop: EventLoop<()>) {
                     delta: Vec2::new(delta.0 as f32, delta.1 as f32),
                 });
             }
+            event::Event::Suspended => {
+                active = false;
+            }
+            event::Event::Resumed => {
+                active = true;
+            }
             event::Event::MainEventsCleared => {
                 handle_create_window_events(
                     &mut app.world,
                     event_loop,
                     &mut create_window_event_reader,
                 );
+                if active {
                     app.update();
                 }
+            }
             _ => (),
         }
     };