判断是否是高清屏设备
var isRetina = function(){
var mediaQuery = "(-webkit-min-device-pixel-ratio: 1.5),\
(min--moz-device-pixel-ratio: 1.5),\
(-o-min-device-pixel-ratio: 3/2),\
(min-resolution: 1.5dppx)";
if (window.devicePixelRatio > 1)
return true;
if (window.matchMedia && window.matchMedia(mediaQuery).matches)
return true;
return false;
};