Faking an undo step for functions that aren't undoable in Gimp

[inkdolls]

Of course I've tested it with Gimp 2.10. Forward compatibility hasn't been an issue. Even complied plugins written in C work across  versions, I'm only using Linux though. Gimp 3 will surely break everything, but that's expected.

set_pixel() wouldn't work (edit: I mean as storage) as it also isn't an undoable operation, it even says so in the description.

Thanks .