[Cin] BlueBanana and alpha slider

[Andrea paz]

In other emails, Igor Vladimirsky identified the commit after which
the alpha slider is no longer functional:
https://git.cinelerra-gg.org/git/?p=goodguy/history.git;a=commit;h=259e435484d99cf6517290f530ac2d9d0d86cd16
If anyone can figure out where the problem lies, they are welcome.

Perhaps in bluebanacolor.c, the line:

float a = 1.f - *S*F;

Which became:

float a = *S*F;

?