title: How to set sample_aspect_ratio while muxing a video stream?
body: <p>I am trying to mux a video stream and an audio stream into an mp4 file, whereas the target file has an inappropriate sar (unlike the source one).</p>
  
  <p>The following methods don't make any sense.
  1、Using avcodec_copy_context(). It cannot set the target sar as the source.</p>
  
  <p>2、Setting sar directly of the target stream. The target sar is not the one I've set in the video stream before writeHeader.</p>
  
  <p>The version I used is 2.8.1.</p>