[duim,gtk] Migrate do-copy-area

When the from-medium and to-medium coincide follow the example in the migration
guide: https://developer.gnome.org/gtk3/3.5/gtk-migrating-2-to-3.html

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I'm not sure about the coordinates when the medium are not the same. I think the original code was also buggy (see the double `get-gcontext(from-medium)`)