title: ffmpeg output redirect to file is buffered and not after every line
body: <p>I'm trying to output the stderr output of ffmpeg to a file as an example...</p>
  <p><code>ffmpeg -i sample.mp4 -vf scale='bitand(oh*dar,65534)':'min(720,ih)' -c:v libx264 -crf 18 -preset veryslow -c:a aac -y sample1.mp4 2&gt;output.log</code></p>
  <p>I notice that the output.log file is written to only in chunks and not every line. How do I get the ouput to the file every line-by-line so I can then process that file and get progress information in realtime? Currently it outputs first few lines and then waits and dumps everything only at the end.</p>