use devicePixelRatio for thumbs in demo
use devicePixelRatio for thumbs in demo

fixes https://github.com/brion/ogv.js/issues/254

chrislearn/ogv.js,paladox/ogv.js,paladox/ogv.js,xzf158/ogv.js,chrislearn/ogv.js,xzf158/ogv.js,paladox/ogv.js,paladox/ogv.js,chrislearn/ogv.js,paladox/ogv.js,xzf158/ogv.js,xzf158/ogv.js,chrislearn/ogv.js,chrislearn/ogv.js,xzf158/ogv.js