PATCH: Small Scroller performance enhancement

Scroller#_fnDrawCallback() forces browsers layout twice, but only needs to do it once with a very simple change. It saves about 30ms for me in IE, which is a lot if you're filtering rows as the user types.

	"_fnDrawCallback": function ()
	{
		var
			that = this,
// NOTE: scrollTop forces browser layout
			iScrollTop = this.dom.scroller.scrollTop,
			iScrollBottom = iScrollTop + this.s.viewportHeight,
// CHANGE: Calculate iTableHeight here while the layout is still valid
			iTableHeight = $(this.s.dt.nTable).height();
		
		/* Set the height of the scrolling forcer to be suitable for the number of rows
		 * in this draw
		 */
		this.dom.force.style.height = (this.s.rowHeight * this.s.dt.fnRecordsDisplay())+"px";
		
		/* Calculate the position that the top of the table should be at */
		var iTableTop = (this.s.rowHeight*this.s.dt._iDisplayStart);
		if ( this.s.dt._iDisplayStart === 0 )
		{
			iTableTop = 0;
		}
		else if ( this.s.dt._iDisplayStart === this.s.dt.fnRecordsDisplay() - this.s.dt._iDisplayLength )
		{
			iTableTop = this.s.rowHeight * this.s.dt._iDisplayStart;
		}

		this.dom.table.style.top = iTableTop+"px";

		/* Cache some information for the scroller */
		this.s.tableTop = iTableTop;
// CHANGE: Use the precalculated iTableHeight
		// this.s.tableBottom = $(this.s.dt.nTable).height() + this.s.tableTop;
		this.s.tableBottom = iTableHeight + this.s.tableTop;

		this.s.redrawTop = iScrollTop - ( (iScrollTop - this.s.tableTop) * this.s.boundaryScale );
		this.s.redrawBottom = iScrollTop + ( (this.s.tableBottom - iScrollBottom) * this.s.boundaryScale );

		if ( this.s.trace )
		{
			console.log(
				"Table redraw. Table top: "+iTableTop+"px "+
				"Table bottom: "+this.s.tableBottom+" "+
				"Scroll boundary top: "+this.s.redrawTop+" "+
				"Scroll boundary bottom: "+this.s.redrawBottom+" "+
				"Rows drawn: "+this.s.dt._iDisplayLength);
		}

		/* Because of the order of the DT callbacks, the info update will
		 * take precidence over the one we want here. So a 'thread' break is
		 * needed
		 */
		setTimeout( function () {
			that._fnInfo.call( that );
		}, 0 );

		/* Restore the scrolling position that was saved by DataTable's state saving
		 * Note that this is done on the second draw when data is Ajax sourced, and the
		 * first draw when DOM soured
		 */
		if ( this.s.dt.oFeatures.bStateSave && this.s.dt.oLoadedState !== null &&
			 typeof this.s.dt.oLoadedState.iScroller != 'undefined' )
		{
			if ( (this.s.dt.sAjaxSource !== null && this.s.dt.iDraw == 2) ||
			     (this.s.dt.sAjaxSource === null && this.s.dt.iDraw == 1) )
			{
				setTimeout( function () {
					$(that.dom.scroller).scrollTop( that.s.dt.oLoadedState.iScroller );
					that.s.redrawTop = that.s.dt.oLoadedState.iScroller - (that.s.viewportHeight/2);
				}, 0 );
			}
		}
	},