libservo: Allow embedders to signal when the cursor has left the WebView (#37317)

Currently, the hover state will stay when the mouse moves out of the
webview, this PR fixes it

Testing: Hover on the `About` on servo.org, and then move the mouse up
to the browser UI, see the hover state resets

Signed-off-by: Tony <legendmastertony@gmail.com>

components/script/script_thread.rs

@@ -1127,6 +1127,14 @@ impl ScriptThread {
                         can_gc,
                     );
                 },
+                InputEvent::MouseLeave(_) => {
+                    self.topmost_mouse_over_target.take();
+                    document.handle_mouse_leave_event(
+                        event.hit_test_result,
+                        event.pressed_mouse_buttons,
+                        can_gc,
+                    );
+                },
                 InputEvent::Touch(touch_event) => {
                     let touch_result =
                         document.handle_touch_event(touch_event, event.hit_test_result, can_gc);