Fix phantom key presses in winit on focus change (#13299) (#13696)

# Objective

Fixes #13299

On Linux/X11, changing focus into a winit window will produce winit
KeyboardInput events with a "is_synthetic=true" flag that are not
intended to be used. Bevy erroneously passes them on to the user,
resulting in phantom key presses.

## Solution

This patch properly filters out winit KeyboardInput events with
"is_synthetic=true".

For example, pressing Alt+Tab to focus a bevy winit window results in a
permanently stuck Tab key until the user presses Tab once again to
produce a winit KeyboardInput release event. The Tab key press event
that causes this problem is "synthetic", should not be used according to
the winit devs, and simply ignoring it fixes this problem.

Synthetic key **releases** are still evaluated though, as they are
essential for correct release key handling. For example, if the user
binds the key combination Alt+1 to the action "move the window to
workspace 1", places the bevy game in workspace 2, focuses the game and
presses Alt+1, then the key release event for the "1" key will be
synthetic. If we would filter out all synthetic keys, the bevy game
would think that the 1 key remains pressed forever, until the user
manually presses+releases the key again inside bevy.

Reference:
https://docs.rs/winit/0.30.0/winit/event/enum.WindowEvent.html#variant.KeyboardInput.field.is_synthetic
Relevant discussion: rust-windowing/winit#3543

## Testing

Tested with the "keyboard_input_events" example. Entering/exiting the
window with various keys, as well as changing its workspace, produces
the correct press/release events.

crates/bevy_winit/src/state.rs

@@ -223,16 +223,27 @@ impl<T: Event> ApplicationHandler<T> for WinitAppRunnerState<T> {
                 );
             }
             WindowEvent::CloseRequested => self.winit_events.send(WindowCloseRequested { window }),
-            WindowEvent::KeyboardInput { ref event, .. } => {
-                if event.state.is_pressed() {
-                    if let Some(char) = &event.text {
-                        let char = char.clone();
-                        #[allow(deprecated)]
-                        self.winit_events.send(ReceivedCharacter { window, char });
+            WindowEvent::KeyboardInput {
+                ref event,
+                is_synthetic,
+                ..
+            } => {
+                // Winit sends "synthetic" key press events when the window gains focus. These
+                // should not be handled, so we only process key events if they are not synthetic
+                // key presses. "synthetic" key release events should still be handled though, for
+                // properly releasing keys when the window loses focus.
+                if !(is_synthetic && event.state.is_pressed()) {
+                    // Process the keyboard input event, as long as it's not a synthetic key press.
+                    if event.state.is_pressed() {
+                        if let Some(char) = &event.text {
+                            let char = char.clone();
+                            #[allow(deprecated)]
+                            self.winit_events.send(ReceivedCharacter { window, char });
+                        }
                     }
+                    self.winit_events
+                        .send(converters::convert_keyboard_input(event, window));
                 }
-                self.winit_events
-                    .send(converters::convert_keyboard_input(event, window));
             }
             WindowEvent::CursorMoved { position, .. } => {
                 let physical_position = DVec2::new(position.x, position.y);