Heterogeneous DPI scaling support
This allows HydraPaper to generate a suitable wallpaper on heterogeneous display scaling (e.g. one display at 200% and the other at 100%).
It works by using globally the highest scaling value found.
I tested this on GNOME Shell 40 Wayland session, with 2 monitors on various orientations and scaling values, including fractional ones, and I did not witness any regressions.
This should fix #16 (closed)