Add a --passthrough-all option to ld-disc-stacker

[jonpoeschek]

I have 5 copies of a laserdisc. When examining the discs in ld-analyse, it's pretty common for there to be a visible dropout that's present on all or most discs, but only one or two of the sources has the dropout marked correctly. This, of course, results in the disc stacker returning the dropout because it's unmarked on most sources and 'good pixels' contain the dropout.

Describe the solution you'd like

I would like an option in ld-disc-stacker called something like --passthrough-all which keeps all dropouts from all discs so ld-dropout-correct can be used to conceal these dropouts.