subtract wave front subtracts the wrong wavefront

[gr5]

I know exactly what is wrong and haven't even looked at the faulty code yet.

I was playing with DFTF and confirmed the bug and then remembered something. A few months ago (roughly) I remember Julien found a bug with the code in the subtractWavefronts code. The code created a list of wavefronts to display in the dialog so you could choose which one to subtract.

But there was a line of code that tried to remove the currently selected wavefront. But it had a bug. So the current wavefront was included.

This function returns the index into the list for which wavefront to subtract from the selected wavefront.

After julien fixed the code (or did I do it? I think Julien), the list properly removed the currently selected wavefront so you can't select the current wavefront from itself. But now the returned index is wrong. This should be an easy fix.

[gr5]

Julien made the change july 20 2025 commit a8ec143
the bug made it into v7.4.0. I had tested this feature quite a big but never subtracted a lower on the list wavefront from a higher one so never noticed the problem.

I have a fix. Will test it and then create a PR.