Keep ratio during full screen modus

use padding to keep image/dom correct ratio

canvas.style.height = window.screen.height + 'px';
                    canvas.style.width = null;
                    var correct_padding = window.screen.width - Math.round(window.screen.height * ratio);
                    canvas.style.paddingLeft = correct_padding / 2 + 'px';
                    canvas.style.paddingRight = correct_padding / 2 + 'px';