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

[Andrea paz]

I tried all 30 patchbay overlays. 17 out of 30 are wrong. I attach the
2 sources used (rosso.png and verde.png) plus the project xml if
anyone wants to repeat the tests. I also attach the screeshots of the
17 wrong overlays so you can compare them with the ones given in the
manual.
You can see that the overlapping area is almost always right. The
problem comes up with the non-overlapping parts, I don't understand
why. You have to do the calculations to figure out what the right
result is and why you don't get it....
I remember the RGB values of green and red that I used as sources (I
sampled them from the "normal" image in the manual):

green: 0.1020  0.8078  0.2980

red:     0.7333  0.1686  0.2980
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