title: CMVideoFormatDescription extensions for MPEG4 streams
body: <p>I've managed to decode and play H264 videos, however I'm having a difficult time with MPEG4 videos.</p>
  
  <p>What CMVideoFormatDescription extensions does it need? I'm getting <code>-8971</code> error (codecExtensionNotFoundErr) when trying to create a VTDecompressionSession. </p>
  
  <p>This is how I create a VideoFormatDescription</p>
  
  <pre><code>OSStatus success = CMVideoFormatDescriptionCreate(kCFAllocatorDefault,
                                                    self.mediaCodec,
                                                    message.frameSize.width,
                                                    message.frameSize.height,
                                                    NULL,
                                                    &amp;mediaDescriptor);
  </code></pre>
  
  <p>Instead of that NULL, I assume I need to specify a CFDictionaryRef, however I don't know what it should contain. Any idea?</p>