lame: Use libav wrapper to encode mp3lame

lame: Use libav wrapper to encode mp3lame

Fixes https://forum.handbrake.fr/viewtopic.php?f=12&t=33345
Some players expect each packet to start on an mp3 frame header. Our
mp3lame encoder did not ensure this and resulted in failure to play
audio on these players.

libav already has the necessary code to parse headers and
accumulate a full frame of data, so use it.