Generate one audio frame per NES frame.

 - Synchronizing audio frames to NES frames simplifies the code a bit and
   makes it easier to reverse audio during rewind. Increasing the number
   of audio frames from the old value of 50 to ~60 shouldn't hurt.

 - Piggyback:

   * Fudge playback rate each frame. Doing it every eight frame
     no longer seems to make a difference.

   * Use separate functions for drawing a frame and handling UI
     keys in the emulation thread.

   * Move audio initialization functions to the end of
     audio.cpp like in other files.