af_scaletempo: optimize overlap searchHEADmaster

scaletempo2 has this optimization where it first uses a step size of 5
together with a quadratic interpolation to quickly get the approximate
position of the best overlap and then does a more thorough search aroun
that area.

Doing the same thing in scaletempo brought a 4.8x performance
improvement, however in my measurements a step size of 3 more
consistently finds good overlaps and it's still a 2.9x improvement for
this function.
I should note that while a step size of 3 produced better numbers,
I was not actually able to hear any difference in my test.

A step size of 3 was chosen just in case it actually makes an audible
difference in some cases and the cpu usage isn't really a problem
anymore, but that can be revisited in the future.

scaletempo2 is still faster then scaletempo with a step size of 5,
which I suspect is mostly because it uses some vectorized functions and
scaletempo does not.

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