prototype change; it looks like we won't need it after all.
  * ./src/audio_output/output.c: if audio output is too slow, but we
    know there is no next packet, then we play it anyway. Rationale behind
    this is: better play a slightly wrongly dated packet than play silence,
    because it might be wrongly dated but there are chances it won't break
    audio continuity.
  * ./modules/audio_output/sdl.c: workaround for SDL's crap audio output
    system (callback gets called at random times, and no way to know the
    latency of the samples being played). Audio output is now rather good
    here, but please give feedback.
  * ./modules/audio_output/oss.c: we delay a bit before asking for a new
    buffer to avoid starvation.