Disable all rasterizer work while backgrounded on iOS (flutter#4002)

On iOS, backgrounded applications are not permitted to execute GPU work.
See [1].  When backgrounding the application, we now tear down the
underlying iOS render surface, and re-create when the app returns to
foreground.

See [2] for a Technica Q&A on GL-related crashes when rendering is
attempted by a backgrounded application.

[1]: https://developer.apple.com/library/content/documentation/3DDrawing/Conceptual/OpenGLES_ProgrammingGuide/ImplementingaMultitasking-awareOpenGLESApplication/ImplementingaMultitasking-awareOpenGLESApplication.html#//apple_ref/doc/uid/TP40008793-CH5-SW1
[2]: https://developer.apple.com/library/content/qa/qa1766/_index.html

shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -313,6 +313,7 @@ - (void)loadView {
 - (void)surfaceUpdated:(BOOL)appeared {
   FTL_CHECK(_platformView != nullptr);
 
+  // NotifyCreated/NotifyDestroyed are synchronous and require hops between the UI and GPU thread.
   if (appeared) {
     _platformView->NotifyCreated();
   } else {
@@ -377,11 +378,14 @@ - (void)applicationWillResignActive:(NSNotification*)notification {
 
 - (void)applicationDidEnterBackground:(NSNotification*)notification {
   TRACE_EVENT0("flutter", "applicationDidEnterBackground");
+  [self surfaceUpdated:NO];
   [_lifecycleChannel.get() sendMessage:@"AppLifecycleState.paused"];
 }
 
 - (void)applicationWillEnterForeground:(NSNotification*)notification {
   TRACE_EVENT0("flutter", "applicationWillEnterForeground");
+  if (_viewportMetrics.physical_width)
+    [self surfaceUpdated:YES];
   [_lifecycleChannel.get() sendMessage:@"AppLifecycleState.inactive"];
 }