eframe: Only run_return twice on Windows (#3053)

The approach of #1889 may remove observers in a view
twice, which produces the Obj-C Exception:

    Cannot remove an observer <...> for the key path
    "nextResponder" from <WinitView ...> because
    it is not registered as an observer.

The above message can only be seen when attaching the
application to debugger. Users normally see:

    [1]    *** trace trap  cargo run

This commit fixes it by only running `event_loop.run_return()`
twice on Windows. Besides:

* We have set `ControlFlow::Exit` on `Event::LoopDestroyed`,
  `EventResult::Exit` and on error; therefore, it is safe
  to not calling `set_exit()`.
* This commit also fix the persistence function in macOS.
  It can't store the content in Memory due to this exception.

Fixed: #2768 (eframe: "App quit unexpectedly" on macOS)

Signed-off-by: pan93412 <pan93412@gmail.com>

crates/eframe/src/native/run.rs

@@ -238,9 +238,15 @@ fn run_and_return(
 
     // On Windows this clears out events so that we can later create another window.
     // See https://github.com/emilk/egui/pull/1889 for details.
-    event_loop.run_return(|_, _, control_flow| {
-        control_flow.set_exit();
-    });
+    //
+    // Note that this approach may cause issues on macOS (emilk/egui#2768); therefore,
+    // we only apply this approach on Windows to minimize the affect.
+    #[cfg(windows)]
+    {
+        event_loop.run_return(|_, _, control_flow| {
+            control_flow.set_exit();
+        });
+    }
 
     returned_result
 }