Make samples per transform configurable

[GWBasic]

Currently, there is one Fourier transform per sample. See if there is a way to have a single Fourier transform shared among a few samples.

Even if the quality is poor, this might be a good way to preview results quickly.

[GWBasic]

TODOs:

• Update options.md
• Write out the samples per transform in the console
• Write out the total time it took to do the upmix

[GWBasic]

This is more challenging than I thought. There are too many corner cases with start and end to do this correctly.

The first branch that I created, 16-make-samples-per-transform-configurable, just won't work. I got caught up in the start and end corner cases. panning_averager never worked when samples per transform was 2; the start and end corner cases are just too confusing at the moment to understand why.

In order to do this, the start and end corner cases need to be cleaned up.

• The TransformedWindowAndPans struct needs is_first and is_last fields
• panning_averager's handling for start and end cases needs to be easier to follow