Selectable: Account for scrolling on the helper's initial position.  Fix...

...ed #8915 - Selectable: lasso offset issue with scrolling

I verified this fix does work but I need some help figuring out how I can write a test for this.  What I have so far is:

	var el = $( "#selectable1").selectable();
	el.simulate( "mousedown", {
		clientX: 10,
		clientY: 10
	});

	$( window ).scrollTop( 1 ).scrollLeft( 1 );

	var helper = $( ".ui-selectable-helper" );
	ok( helper.css( "top" ), '11px' );  // Fails because the top is '10px'
	ok( helper.css( "left" ), '11px' );  // Fails because the left is '10px'

The problem is neither  `scrollTop` nor `scrollLeft` will actually change the the scroll position unless the window is actually scrollable.  I can't think of a way to do this without mocking `scrollTop` and `scrollLeft`.  Ideas?