Only resize if content is visible

Seen on FF:

    <div style="display:none">
        <iframe src="somethingUsing_xLayout.html"></iframe>
    </div>

Now somethingUsing_xLayout.html will have e.g. $(window).height() == 0,
elements have undefined margin-top's and other strange things.

That causes xLayout to fail, so a check is made to see if _this_ is
visible.