[Cin] possible alpha bug in processing

[Igor BEGHETTO]

> the render with PNG, EXR, and TGA which I "think" was already working 
> correctly and Andrew confirmed.
Yes. I had tested that thing without Andrew's mod. Sorry.

Thanks Andrew for the fix 
(0001-background-clear-in-vconsole-must-be-set-to-0.patch).
Thanks Phyllis for the Ubuntu16-AppImage and for the note in 
MantisBT0000558. I just noticed that the dimension of 
"test_ubuntu16.AppImage" is 82MByte while 
"CinGG-20221031-x86_64-older_distros.AppImage" is 137MByte. It is weird. 
Is there a reason?

I hope RafaMar and other users may test it with their project before the 
next Cinelerra-GG release.

IgorBeg