Thumbnail orientation wrong

With some videos (for instance from the iPhone) if the user is recording in portrait the thumbnail will be created with the wrong orientation.

FFmpeg can get the rotation from the file metadata, and we can use this to pass the correct filter parameters in the thumbnail generation code.

The attached patch fixes this for me. I've also attached a sample movie where this is happening.

ps. This might require a recent version of ffmpeg. I'm running 1.1.3.