When a gtk_widget_queue_allocate() on some widget increases the clip,
widget->parent's clip was not updated. This appraoch naively just
unions widget's new clip with widget->parent's clip.

This of course only works if widget and parent share the same GDK
window. In the cases where they don't we can't do anything and need a
better fix.

Fixes label-text-shadow-changes-modify-clip.ui reftest.