其他参考链接：
1、https://www.cnblogs.com/smartsensor/p/4343769.html
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/*Set the video scaling factor。
Zero is a special value; it will adjust the video to the output
window/drawable (in windowed mode) or the entire screen.
*/
设置缩放系数
void libvlc_video_set_scale( libvlc_media_player_t *p_mi, float f_factor );
/*Set new video aspect ratio.*/
设置视频长宽比
void libvlc_video_set_aspect_ratio( libvlc_media_player_t *p_mi, const char *psz_aspect );