widget: Correctly handle which widgets to include in the clip

Previously, we would not include any child widget on the first
allocation, which happens right after realize(), but before map(). No
widget is drawable at that point.

gtk/gtkwidget.c

@@ -15578,7 +15578,8 @@ union_with_clip (GtkWidget *widget,
 {
   GtkAllocation widget_clip;
 
-  if (!gtk_widget_is_drawable (widget))
+  if (!gtk_widget_is_visible (widget) ||
+      !gtk_widget_get_child_visible (widget))
     return;
 
   gtk_widget_get_clip (widget, &widget_clip);