title: FFmpeg -ss weird behaviour
body: <p>I've been using FFmpeg to extract single frames into an image. Though some Googling, it turns out that running:</p>
  
  <pre><code>ffmpeg.exe -i video.avi -ss 00:30:00 -y -an -vframes 1 test.png
  </code></pre>
  
  <p>...runs SIGNIFICANTLY slower than the following, which is nearly identical, but instantaneous:</p>
  
  <pre><code>ffmpeg.exe -ss 00:30:00 -i video.avi -y -an -vframes 1 test.png
  </code></pre>
  
  <p>The only difference is the order of -i and -ss. Is this an intentional 'feature'? Is there some sort of technical reason for the difference here?</p>