Tweak thumbnail/poster usage
Tweak thumbnail/poster usage

fixes https://github.com/brion/ogv.js/issues/271
fixes https://github.com/brion/ogv.js/issues/256

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