Bug#851359: diffoscope: Improve support for comparing images

[Chris Lamb]

Hey Maria,

> I believe the attached patch provides the requested feature.

Oh, awesome!

> I would be very grateful for any review or general advice on how to make
> it better.

Could you push this to a branch so we can more easily play with it? :)


Best wishes,

-- 
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     : :'  :     Chris Lamb
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