从CSS实现正片叠底看=>混合模式mix-blend-mode

兼容性：这个东西说多了也没意思，像HTML5和CSS3这种兼容性时刻变化的东东，我们最好在自己支持的设备上实验，不支持，就在想办法呗，这个东西就是为了方便和好玩

所有属性：

mix-blend-mode: normal;          //正常
mix-blend-mode: multiply;        //正片叠底
mix-blend-mode: screen;          //滤色
mix-blend-mode: overlay;         //叠加
mix-blend-mode: darken;          //变暗
mix-blend-mode: lighten;         //变亮
mix-blend-mode: color-dodge;     //颜色减淡
mix-blend-mode: color-burn;      //颜色加深
mix-blend-mode: hard-light;      //强光
mix-blend-mode: soft-light;      //柔光
mix-blend-mode: difference;      //差值
mix-blend-mode: exclusion;       //排除
mix-blend-mode: hue;             //色相
mix-blend-mode: saturation;      //饱和度
mix-blend-mode: color;           //颜色
mix-blend-mode: luminosity;      //亮度
mix-blend-mode: initial;         //初始
mix-blend-mode: inherit;         //继承
mix-blend-mode: unset;           //复原