[Cin] Some tests on blending

[Andrea paz]

I append new plugins.text

> I wonder if you think parts of it should be included in the manual too but so far it seems already covered.

I wouldn't really know what to put in.

PS: I saw the manual update you mentioned in a previous mail about my
attempts at Gimp-like functions. They are not very important and do
not serve to solve cases where our overlays work differently from
CinCV's. Besides, the tests I did are not representative, as is
evident from the thread. I would remove that commit...
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