Sets the widget’s clip. This must not be used directly, but from within a widget’s size_allocate method. It must be called after Widget.setAllocation (or after chaining up to the parent class), because that function resets the clip.
The clip set should be the area that widget draws on. If widget is a gtk.Container, the area must contain all children's clips.
If this function is not called by widget during a ::size-allocate handler, the clip will be set to widget's allocation.