[PD] [GEM] Flatten Alpha for storage, then restore it

[Spencer Russell]

I'm mixing video by background subtracting it and then setting the
background transparent (alpha = 0), then texturing each video onto
overlapping rectangles to achieve a compositing effect.

I need to record the different videos with [pix_record] and then play
them back later while preserving the alpha somehow, so I was thinking
that I could just replace all the transparent part with bright pink or
something and store it as regular photo jpeg frames, and then later
set all the bright pink parts to transparent when I read the video
back.

I'm having trouble with both sides of the conversion, though. Perhaps
there's a better approach entirely?

thanks,
spencer