Use better rounded divisions

[gtarsia]

This change makes raising/lowering/setting a percent more consistent.

It fixes #175 where, at certain percents, increasing by 1 causes the brightness to flicker or just get stuck.

It does so by rounding the brightness percent to closest integer.

[gtarsia]

Ok, renamed div_round to div_round_u32.

I think this is good to go, switching brightness now looks great on my setup.

Btw, I see this warning when changing brightness (which was there before my changes):

(swayosd-server:55525): Gtk-WARNING **: 10:58:59.635: GtkGizmo 0x557ce3f4e9f0 (progress) reported min width -2, but sizes must be >= 0

and, not only I don't understand what's causing it, I couldn't make any quick fixes I found online work, such as:

• Setting min-width: 0 to progressbar in .config/swayosd/style.css
• Calling set_size_request (progress_widget, 0, -1) on the progress bar.

[ErikReider]

Btw, I see this warning when changing brightness (which was there before my changes):

(swayosd-server:55525): Gtk-WARNING **: 10:58:59.635: GtkGizmo 0x557ce3f4e9f0 (progress) reported min width -2, but sizes must be >= 0

Might be your custom CSS? Not seeing it on my end...

[ErikReider]

LGTM, thanks! :)