[Cin] Fade In/Fade Out flash bug

[Andrea paz]

In MantisBT #0559, Georgy took an in-depth look at how Dissolve works
when the two affected clips have an alpha channel at different opacity
(see comment 005645, for a detailed description). The purpose is to
eliminate The flash of gray lettering just before having the normal
Dissolve. Georgy explained the problem well.
A patch appeared in CinHV to fix the problem but Georgy saw no
difference between CinGG and CinHV with the same project (provided by
IgorBeg). Investigating further he saw that the normal operation of
Dissolve leads to the very flash we considered a bug. So the question
is whether to leave it as is or make interventions to have a different
result, more in line with expectations. Do you have any
ideas/proposals on how Dissolve/Fade should work?

https://www.cinelerra-gg.org/bugtracker/view.php?id=559