code: 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]
          });
      }
docs: Positions the overlay elements: top, middle & bottom.