* Add first cut of code to handle Windows Media Player rate switching

  requests. The current state is that at startup, WMP will get the
  best stream that it can handle. However, subsequent rate switching
  only puts a message in the log saying what the new stream ought to
  be. Solving this will be tricky. I guess that we would have to wait for
  key frames to appear in the new stream, and then switch over to it.
  Some care would be needed to deal with the PTS of the new stream
  versus the old stream.

Originally committed as revision 602 to svn://svn.ffmpeg.org/ffmpeg/trunk