"Reverse" --scale-down (zoom images to screen size)

[derf]

Requested by mail:

Don't go fullscreen, but do make the window as big as possible (either max out width or height) and zoom the image in to fit if necessary.
So, it's like --scale-down, but zooming in, not out.

This can already partially be achieved by combining --auto-zoom and --geometry / the "Freeze Window Size" menu setting.

[derf]

Well, nope, not gonna happen