Returns an approximation of the clip returned by getClip, as
an axis-aligned Rect. This is used by the semantics layer to
determine whether widgets should be excluded.
Called whenever a new instance of the custom clipper delegate class is
provided to the clip object, or any time that a new clip object is created
with a new instance of the custom clipper delegate class (which amounts to
the same thing, because the latter is implemented in terms of the former).