layout() {

		// Layout slides
		this.Reveal.getHorizontalSlides().forEach( ( hslide, h ) => {
			hslide.setAttribute( 'data-index-h', h );
			transformElement( hslide, 'translate3d(' + ( h * this.overviewSlideWidth ) + 'px, 0, 0)' );

			if( hslide.classList.contains( 'stack' ) ) {

				queryAll( hslide, 'section' ).forEach( ( vslide, v ) => {
					vslide.setAttribute( 'data-index-h', h );
					vslide.setAttribute( 'data-index-v', v );

					transformElement( vslide, 'translate3d(0, ' + ( v * this.overviewSlideHeight ) + 'px, 0)' );
				} );

			}
		} );

		// Layout slide backgrounds
		Array.from( this.Reveal.getBackgroundsElement().childNodes ).forEach( ( hbackground, h ) => {
			transformElement( hbackground, 'translate3d(' + ( h * this.overviewSlideWidth ) + 'px, 0, 0)' );

			queryAll( hbackground, '.slide-background' ).forEach( ( vbackground, v ) => {
				transformElement( vbackground, 'translate3d(0, ' + ( v * this.overviewSlideHeight ) + 'px, 0)' );
			} );
		} );

	}
Uses CSS transforms to position all slides in a grid for
display inside of the overview mode.