[theora] Better A/V sync and framerate detection in ffmpeg2theora

[Jason Self]

In addition to the indexing mentioned earlier by Chris Pearce,  
improvements to frame rate detection and A/V sync were recently  
committed to ffmpeg2theora. This will help with variable framerate  
video and in other cases where the container specifies the incorrect  
framerate.

Remember those cases where it would report the framerate as double  
what it really was? That seems to be fixed now. In my testing,  
ffmpeg2theora should now keep lip-synch as well as any DVD player can.

https://trac.xiph.org/changeset/16650