[Cin] oops apparently 'subtract' mode is broken!

[Phyllis Smith]

Preliminary results show Subtract (Difference) mode is working same as in
this url as Andrew quoted.  See attached where compare3.gif shows the URL's
difference with Cinelerra subtract below and input.gif shows CinGG input
files.

> according to this
>
> https://emptyeasel.com/2008/11/14/layer-modes-part-three-difference-addition-subtraction-darken-lighten/
>
> color inversion in subtract mode IS normal thing.
>
> yet in original Cin something else happens ....
>
>
> may be Phyllis can replace our original red-green square  images with
> full-color gradients + photo, so effect will be more natural? Or add this
> into new appendix .....
>
>
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