Mainly, this fixes auto UI scaling with _single-monitor_ fractional
setups (see the comment in `display_server_wayland.cpp` for more info).
This is the result of a bunch of current limitations, mainly the fact
that the UI scale is static (it's probed at startup) and the fact that
Wayland exposes fractional scales only at the window-level, by design.
The `screen_get_scale` special case should help in 99% of cases, while
the auto UI scale part will unfortunately only help with single-screen
situations, as multi-screen fractional scaling requires dynamic UI
scale changing.
1bb8199342