LibGUI: Only redraw the dirty rect in GWidget.

There is some trouble here with the asynchronous nature of WindowServer
and the single per-window backing store we're drawing into. If we start
repainting a widget with a pending invalidation, that invalidation might
get flushed by the WindowServer mid-paint.

LibGUI/GWidget.cpp

@@ -61,7 +61,7 @@ void GWidget::paintEvent(GPaintEvent& event)
 {
     if (fillWithBackgroundColor()) {
         Painter painter(*this);
-        painter.fill_rect(rect(), backgroundColor());
+        painter.fill_rect(event.rect(), backgroundColor());
     }
     for (auto* ch : children()) {
         auto* child = (GWidget*)ch;