. fixed audio syncword detection for mpeg 1 streams (hopefully)
    FIXME: can anyone confirm we can get rid of adec_byte_stream_next
    in audio_decoder_thread.c now that we have the new input ?

 . fixed Layer 2 Mono decoding.
    FIXME: who created freq_table ? what was the rationale for it, and
    where does it come from ?
    FIXME: how can we spare the extra DCT in audio_decoder.c:707 ? I'm too
    tired to think about it.

 . proposal: splitting audio_decoder.c in one file per layer.