bug#57990: Add package: python-mat2 (remove metadata from images to improve privacy)

[Tobias Geerinckx-Rice]

> can I express "any version of ffmpeg"?

No, this would go against the goals of Guix: packages can't depend on 
properties of the environment they'll end up in (if any).

They must be self-contained, or use standard features like $PATH et al to find 
optional dependencies at run time, but these will have to be provided by the 
user/manifest/whatever.

Sorry,

T G-R

Sent on the go.  Excuse or enjoy my brevity.