Ristretto MR #8 - Scaling corrections

[ML]

Hi,
if anyone have the time, I'd like a review of the MR and some testing.

https://gitlab.xfce.org/apps/ristretto/-/merge_requests/8

I refactored the scaling calculations to fix the issues mentioned in the MR.
With that new scaling limits are 10% minimum of the ristretto window,
and maximum is 150%.

Thanks.