[Cin] Some tests on blending

[Andrea paz]

Obviously it was my fault: I had not removed the clip in Blend Program....
I re-tested and most of the color-related plugins do not have
clipping. Unfortunately (at least for me) some of the most important
plugins have it:

Blur
Color 3 Way
Color Space
Foreground
Histogram
Histogram Bezier
Title
Videoscope

I remember an attempt by Andrew with Adam to try to engage him on the
Histogram clipping, but Adam was not really interested:
https://github.com/heroineworshiper/hvirtual/issues/8

@Phyllis
Do you think it's good to indicate in the manual the plugins that do
the clipping?