label: Include all of the allocation in the clip

Otherwise, the CSS background we draw would be clipped if the ink rect
was smaller than the allocation (a very common thing).

Broken since 37030a7710 where we clipped
to the ink rect.

gtk/gtklabel.c

@@ -3996,6 +3996,7 @@ gtk_label_size_allocate (GtkWidget     *widget,
     }
 
   gtk_label_get_ink_rect (label, &clip_rect);
+  gdk_rectangle_union (&clip_rect, allocation, &clip_rect);
   gtk_widget_set_clip (widget, &clip_rect);
 }