Reduce internal system order ambiguities, and add an example explaining them (#7383)

# Objective

- Bevy should not have any "internal" execution order ambiguities. These clutter the output of user-facing error reporting, and can result in nasty, nondetermistic, very difficult to solve bugs.
- Verifying this currently involves repeated non-trivial manual work. 

## Solution

- [x] add an example to quickly check this
- ~~[ ] ensure that this example panics if there are any unresolved ambiguities~~
- ~~[ ] run the example in CI 😈~~

There's one tricky ambiguity left, between UI and animation. I don't have the tools to fix this without system set configuration, so the remaining work is going to be left to #7267 or another PR after that.

```
2023-01-27T18:38:42.989405Z  INFO bevy_ecs::schedule::ambiguity_detection: Execution order ambiguities detected, you might want to add an explicit dependency relation between some of these systems:
 * Parallel systems:
 -- "bevy_animation::animation_player" and "bevy_ui::flex::flex_node_system"
    conflicts: ["bevy_transform::components::transform::Transform"]
  ```

## Changelog

Resolved internal execution order ambiguities for:
1. Transform propagation (ignored, we need smarter filter checking).
2. Gamepad processing (fixed).
3. bevy_winit's window handling (fixed).
4. Cascaded shadow maps and perspectives (fixed).

Also fixed a desynchronized state bug that could occur when the `Window` component is removed and then added to the same entity in a single frame.

Author: alice-i-cecile

src/lib.rs

@@ -25,8 +25,8 @@ use bevy_utils::{
     Instant,
 };
 use bevy_window::{
-    CursorEntered, CursorLeft, CursorMoved, FileDragAndDrop, Ime, ModifiesWindows,
-    ReceivedCharacter, RequestRedraw, Window, WindowBackendScaleFactorChanged,
+    exit_on_all_closed, CursorEntered, CursorLeft, CursorMoved, FileDragAndDrop, Ime,
+    ModifiesWindows, ReceivedCharacter, RequestRedraw, Window, WindowBackendScaleFactorChanged,
     WindowCloseRequested, WindowCreated, WindowFocused, WindowMoved, WindowResized,
     WindowScaleFactorChanged,
 };
@@ -55,8 +55,11 @@ impl Plugin for WinitPlugin {
                 CoreStage::PostUpdate,
                 SystemSet::new()
                     .label(ModifiesWindows)
-                    .with_system(changed_window)
-                    .with_system(despawn_window),
+                    // exit_on_all_closed only uses the query to determine if the query is empty,
+                    // and so doesn't care about ordering relative to changed_window
+                    .with_system(changed_window.ambiguous_with(exit_on_all_closed))
+                    // Update the state of the window before attempting to despawn to ensure consistent event ordering
+                    .with_system(despawn_window.after(changed_window)),
             );
 
         #[cfg(target_arch = "wasm32")]

src/system.rs

@@ -87,14 +87,18 @@ pub struct WindowTitleCache(HashMap<Entity, String>);
 
 pub(crate) fn despawn_window(
     closed: RemovedComponents<Window>,
+    window_entities: Query<&Window>,
     mut close_events: EventWriter<WindowClosed>,
     mut winit_windows: NonSendMut<WinitWindows>,
 ) {
     for window in closed.iter() {
         info!("Closing window {:?}", window);
-
-        winit_windows.remove_window(window);
-        close_events.send(WindowClosed { window });
+        // Guard to verify that the window is in fact actually gone,
+        // rather than having the component added and removed in the same frame.
+        if !window_entities.contains(window) {
+            winit_windows.remove_window(window);
+            close_events.send(WindowClosed { window });
+        }
     }
 }