WindowServer: Fix compositing of fullscreen window

I honestly don't know the internals of all this and what exactly is
going on, but this fixes compositing of the fullscreen window. By trial
and error I found that specifically m_invalidated_all needs to be set to
false, so it's probably different behaviour in prepare_dirty_rects(),
which depends on that...
Either way, the code composing all windows in non-fullscreen mode calls
Window::clear_dirty_rects() for each, so not doing that for the fullscreen
window as well seems like an oversight.

Fixes #4810.

Userland/Services/WindowServer/Compositor.cpp

@@ -422,6 +422,7 @@ void Compositor::compose()
     if (m_invalidated_window) {
         if (auto* fullscreen_window = wm.active_fullscreen_window()) {
             compose_window(*fullscreen_window);
+            fullscreen_window->clear_dirty_rects();
         } else {
             wm.for_each_visible_window_from_back_to_front([&](Window& window) {
                 compose_window(window);