Continue execution after closing native eframe window (#1889)

This adds `NativeOptions::run_and_return` with default value `true`.

If `true`, execution will continue after the eframe window is closed. This is a new behavior introduced in this PR.
If `false`, the app will close once the eframe window is closed. The is the old behavior.

This is `true` by default, and the `false` option is only there so we can revert if we find any bugs.

When `true`, [`winit::platform::run_return::EventLoopExtRunReturn::run_return`](https://docs.rs/winit/latest/winit/platform/run_return/trait.EventLoopExtRunReturn.html#tymethod.run_return) is used. The winit docs warns of its usage, recommending `EventLoop::run`, but 🤷 
When `false`, [`winit::event_loop::EventLoop::run`](https://docs.rs/winit/latest/winit/event_loop/struct.EventLoop.html#method.run) is used.

This is a really useful feature. You can now use `eframe` to quickly open up a window and show some data or options, and then continue your program after the `eframe` window is closed

My previous attempt at this caused some problems, but my new attempt seems to be working much better, at least on my Mac.

eframe/src/epi.rs

@@ -269,6 +269,20 @@ pub struct NativeOptions {
     ///
     /// Default: `Theme::Dark`.
     pub default_theme: Theme,
+
+    /// This controls what happens when you close the main eframe window.
+    ///
+    /// If `true`, execution will continue after the eframe window is closed.
+    /// If `false`, the app will close once the eframe window is closed.
+    ///
+    /// This is `true` by default, and the `false` option is only there
+    /// so we can revert if we find any bugs.
+    ///
+    /// This feature was introduced in <https://github.com/emilk/egui/pull/1889>.
+    ///
+    /// When `true`, [`winit::platform::run_return::EventLoopExtRunReturn::run_return`] is used.
+    /// When `false`, [`winit::event_loop::EventLoop::run`] is used.
+    pub run_and_return: bool,
 }
 
 #[cfg(not(target_arch = "wasm32"))]
@@ -295,6 +309,7 @@ impl Default for NativeOptions {
             renderer: Renderer::default(),
             follow_system_theme: cfg!(target_os = "macos") || cfg!(target_os = "windows"),
             default_theme: Theme::Dark,
+            run_and_return: true,
         }
     }
 }

eframe/src/lib.rs

@@ -161,20 +161,20 @@ mod native;
 /// ```
 #[cfg(not(target_arch = "wasm32"))]
 #[allow(clippy::needless_pass_by_value)]
-pub fn run_native(app_name: &str, native_options: NativeOptions, app_creator: AppCreator) -> ! {
+pub fn run_native(app_name: &str, native_options: NativeOptions, app_creator: AppCreator) {
     let renderer = native_options.renderer;
 
     match renderer {
         #[cfg(feature = "glow")]
         Renderer::Glow => {
             tracing::debug!("Using the glow renderer");
-            native::run::run_glow(app_name, &native_options, app_creator)
+            native::run::run_glow(app_name, &native_options, app_creator);
         }
 
         #[cfg(feature = "wgpu")]
         Renderer::Wgpu => {
             tracing::debug!("Using the wgpu renderer");
-            native::run::run_wgpu(app_name, &native_options, app_creator)
+            native::run::run_wgpu(app_name, &native_options, app_creator);
         }
     }
 }