[Cin] variable fast/slow playback speed hack

[Phyllis Smith]

I must be wrong about "SCRUB_SPEED, I think it was something the original
author added but never got working.  So maybe the shuttle hardware really
did exist back then.  I will hook up the shuttle and uncomment the code to
see if I can get it to do anything.

>From Audacity,
https://manual.audacityteam.org/man/scrubbing_and_seeking.html

Scrubbing or Seeking is the action of moving the mouse pointer right or
left so as to adjust the position, speed or direction of playback,
listening to the audio at the same time - a convenient way to quickly
navigate the waveform to find a particular event of interest.

The term "Scrubbing" comes from the early days of the recording industry
and refers to the process of physically rotating tape reels to move the
tape past the play head to locate a specific point in the audio track.

In Audacity

   - *Scrubbing* changes the audio position without skipping.
   - *Seeking* changes the audio position, skipping over audio to move more
   quickly
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