title: Ffmpeg didn't burn srt subtitle on mkv properly
body: <p>`ffmpeg version:git-fc6de70 (2013-07-18) with libass and fontconfig support.
  command line:ffmpeg.exe -i test.mkv -vf subtitles=test.srt out.mkv
  and i got this error.:</p>
  
  <pre><code>Metadata:
      encoder         : Lavf55.12.102
      Stream #0:0(eng): Video: h264 (libx264) (H264 / 0x34363248), yuv420p, 1280x5
  76 [SAR 1:1 DAR 20:9], q=-1--1, 1k tbn, 23.98 tbc (default)
      Stream #0:1: Audio: vorbis (libvorbis) (oV[0][0] / 0x566F), 48000 Hz, stereo
  , fltp (default)
  Stream mapping:
    Stream #0:0 -&gt; #0:0 (h264 -&gt; libx264)
    Stream #0:1 -&gt; #0:1 (aac -&gt; libvorbis)
  Press [q] to stop, [?] for help
  [Parsed_subtitles_0 @ 00000000027b9960] Neither PlayResX nor PlayResY defined. A
  ssuming 384x288
  </code></pre>
  
  <p><a href="http://tinypic.com/r/2ag52so/5" rel="nofollow">http://tinypic.com/r/2ag52so/5</a></p>