阿里云oss
poster = src + '?spm=a2c4g.11186623.2.1.yjOb8V&x-oss-process=video/snapshot,t_7000,f_jpg,w_800,h_600,m_fast';
七牛云oss
http://xxx.clouddn.com/test.mp4?vframe/jpg/offset/8/w/300/h/200