[Cin] possible alpha bug in processing

[Andrew Randrianasulu]

User 'goat' reported cingg failed to create png sequence with alpha, even
if proj set to rgba and png settings also include alpha.

I ..spend a lot of tume in filepng and fule, until it dawned at me Ican
check older cingg!

Bug was not there in june 15 2020  selfbuild.

And bug was present in 09 30 2020 ubuntu 16 static tar ...


So I searched git log and found this commit

https://git.cinelerra-gg.org/git/?p=goodguy/cinelerra.git;a=blobdiff;f=cinelerra-5.1/cinelerra/virtualvconsole.C;h=3c3fb86f5710e15954c927f358210027cb819ed1;hp=193761b6763a61d11b0b85c8edbb4ec658df2b99;hb=abdff69b9309c7d5cd2ed6ce17dd2e0d85aef9a1;hpb=0b751b07a28e84a721b2bb76083db6629aa26d73


i tried to set alpha to 0 (zero) in virtialvconsole and suddently pngs and
exrs and tgas got their alpha back!


How to check:

Load video into cingg
Set format to rgba-8 bit
Set Alpha plugin over video, with parameter 0.5
or use fader.
Set output render to one frame, png, use alpha.

Render

While image will be  with right format alpha will be absent as evidenced by
gimp's colorpicker.

patch cingg with attached patch, hope alpha will be back!


But of course we better to re-test video formats with alpha too ... (just
not sure how)

note, ffmpeg's png encoder worked ok, and so did tiff encoder. But
png/exr/tga were all busted wrt alpha.
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