[Cin] User Defined Blend Algebra

[Andrea paz]

I also applied the latest patch and did all the tests with your
examples projects and then tried to redo them with two tracks of my
own: everything seems to work fine even though some tests with my
tracks I don't understand well. :)
Nice examples are also ydiff; swap and transition. The chromakey seems
to me comparable to CGG's classic chromakey and not avid's. The
ability to replace the background via Blend Program is great, though.
I am left with the curiosity of combining Chromakey HSV with Blend
Program's background replacement, but I will put it off until Monday.
Thanks also for the additional insights for the note for the manual;
they are all interesting and make one understand some of the problems
that have come up over the years (and maybe even find a solution via
custom Blend Algebra functions...).
I hope others will also test because I am not good as a tester.