Wish: flicker fixer

[CarlFK]

This isn't a voctomix issue, but close enough.
I sometimes end up with videos with black frames. (due to some mistake I made and the best fix is not to make mistakes.)

That said, it would be lovely to have some code to fix this mistake.
basically: when a feed goes black, us the previous frame.

I would love for voctocore to implement this, except I don't want to pollute core with such stuff, so forget about that.
I suspect sometimes the flicker is caused by bad network or clock skew between client and core, so I'm not sure I can fix it client side.

What I am interested in: fix the resulting mixed video in post. (eww)
implementation:
read the vocto configs to get the pre defined compositions
for each frame, guess which composition is in use.
if a region is black, copy the region from the previous frame.

I can type this issue, but my gst skills aren't quite up to coding this.