[mythtv] Subclassing avfdecoder

[Kevin Kuphal]

I just wanted to let the developers know that I've successfully implemented
a subclass of avfdecoder for MythMusic handling MP3 decoding.  This just
proves a point that it can be trivial to implement a decoder for MythMusic
now for any format supported by libavformat.  This also means that the
current MP3 and Vorbis decoding could be cut at some time as the powers that
be see fit.  The subclass does nothing but call the parent (avfDecoder)
class methods and presents its own proper extension support.

I hope to take a look at encoding with libavformat next.

Kevin