Fix crash when changing viewport settings (#4862)

* Fixes #3959

There are two bugs racing each other here, which is why it sometimes
crashes and sometimes the app just silently exists

Bug 1
When the window is recreated a Destroyed event arrives (due to the Drop
of the old window). The code that receives this event does not look to
see if its the main viewport or a secondary one and unconditionally
closes the app. The code path for other platforms is slightly different
and does check.

I have moved the code that handles the destroy to be in the same place
and have the same behavior as the other platforms.

Bug 2

At recreate time the window and winit entries of the viewport are set to
None (forcin g them to be recreated). But the surface is still bound to
the old window, this causes the next context switch to fail. So I simply
added a viewport.gl_surface = None too,


This is my first egui PR so I hope I have not broken anything. If
nothing else I understand a little better how egui works.

crates/eframe/src/native/glow_integration.rs

@@ -803,6 +803,18 @@ impl GlowWinitRunning {
                 }
             }
 
+            winit::event::WindowEvent::Destroyed => {
+                log::debug!(
+                    "Received WindowEvent::Destroyed for viewport {:?}",
+                    viewport_id
+                );
+                if viewport_id == Some(ViewportId::ROOT) {
+                    return EventResult::Exit;
+                } else {
+                    return EventResult::Wait;
+                }
+            }
+
             _ => {}
         }
 
@@ -1360,6 +1372,7 @@ fn initialize_or_update_viewport(
                 );
                 viewport.window = None;
                 viewport.egui_winit = None;
+                viewport.gl_surface = None;
             }
 
             viewport.deferred_commands.append(&mut delta_commands);