proportional zooming

[rra]

In XNVIEW one can set the step size of the zooming in fixed steps,
these steps are in a percentage of the 100% zoom size of the image.
E.g. a step-size of 20% means:
zoom-out: 100% > 80% > 60 % > 40 % > 20% ...

It would be nice if the zooming steps are relative to the size
of the last image size;
thus: 20% steps would yield:
zoom-out: 100% > 80% > 64% > 51 % > 41% and so on;

René