Positions the overlay elements: top, middle & bottom.
function OverlayLayout(context, options) {
        var height = (context.size[1] - options.itemSize) / 2;
        context.set('top', {
            size: [context.size[0], height],
            translate: [0, 0, 1]
        });
        context.set('middle', {
            size: [context.size[0], context.size[1] - (height * 2)],
            translate: [0, height, 1]
        });
        context.set('bottom', {
            size: [context.size[0], height],
            translate: [0, context.size[1] - height, 1]
        });
    }