function showChalkboard() {
//console.log("showChalkboard");
		clearTimeout( touchTimeout );
		touchTimeout = null;
		drawingCanvas[ 0 ].sponge.style.visibility = 'hidden'; // make sure that the sponge from touch events is hidden
		drawingCanvas[ 1 ].sponge.style.visibility = 'hidden'; // make sure that the sponge from touch events is hidden
		drawingCanvas[ 1 ].container.style.opacity = 1;
		drawingCanvas[ 1 ].container.style.visibility = 'visible';
		mode = 1;
	}
Show an overlay for the chalkboard.