function createScreen() {
		var params    = settings.screen;
		var width     = params.width;
		var height    = params.height;
		var pixelSize = params.pixelSize;

		var texture = pixelbox.$screen = new Texture(width, height);
		if (__USE_WEBGL__) texture.canvas = webGL.context.canvas;
		var canvas = texture.canvas;
		var style  = canvas.style;

		document.body.appendChild(canvas);

		if (params.fullscreen) {
			if (params.keepAspectRatio) {
				function resizeScreen() {
					var iw = window.innerWidth;
					var ih = window.innerHeight;
					var sw = width  * pixelSize.width;
					var sh = height * pixelSize.height;
					var ratio = Math.min(iw / sw, ih / sh);
					var fw = sw * ratio;
					var fh = sh * ratio;
					// get current canvas (allow $screen redefinition, e.g. webGL)
					var style = pixelbox.$screen.canvas.style;
					style.width  = fw + 'px';
					style.height = fh + 'px';
					style.left = (iw - fw) / 2 + 'px';
					style.top  = (ih - fh) / 2 + 'px';
				}
				addEventListener('resize', resizeScreen);
				resizeScreen();
			} else {
				style.width = style.height = '100%';
			}
		} else {
			style.width  = width  * pixelSize.width  + 'px';
			style.height = height * pixelSize.height + 'px';
		}

		return texture;
	}
change default tilesheet used.
@param {Image | Texture | Map} img - tilesheet to use as default.
                        It can be any renderable thing in Pixelbox