eframe: add call to save_and_destroy in Exit event of run_return (#2895)

`EventResult::Exit` is hit before `Event::LoopDestroyed` is, and due to
possibly some order of operations or drops the window is never destroyed
on Linux. Adding a call to `winit_app.save_and_destroy();` where
`EventResult::Exit` is checked solves this.

Closes #2892

Signed-off-by: Luke D. Jones <luke@ljones.dev>

crates/eframe/src/native/run.rs

@@ -190,6 +190,7 @@ fn run_and_return(
             }
             EventResult::Exit => {
                 tracing::debug!("Asking to exit event loop…");
+                winit_app.save_and_destroy();
                 *control_flow = ControlFlow::Exit;
                 return;
             }