xwayland: support HiDPI scale

[moetayuko]

Describe your PR, what does it fix/add?

This supports the xorg-xwayland patch at https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/733

Ported from https://gitlab.freedesktop.org/lilydjwg/wlroots/-/commit/5a7c65cab11c9f3c3dc79f9c4cfb8a6acf6cbaae

See #6281 for motivation.

AUR package hyprland-hidpi-xprop-git applies this patch on top of hyprland mainline. After installing, do the following to enable (scaling factor 2 in this example):

1. Add exec-once = systemctl --user start xsettingsd.service && echo "Xft.dpi: 192" | xrdb -merge && xprop -root -format _XWAYLAND_GLOBAL_OUTPUT_SCALE 32c -set _XWAYLAND_GLOBAL_OUTPUT_SCALE 2 to hyprland configuration.
2. Add the following to ~/.config/xsettingsd/xsettingsd.conf

   Gdk/UnscaledDPI 98304
   Gdk/WindowScalingFactor 2
   

3. Remove GDK_SCALE, QT_SCALE_FACTOR, and other per-application scaling options.

Is there anything you want to mention? (unchecked code, possible bugs, found problems, breaking compatibility, etc.)

N/A

Is it ready for merging, or does it need work?

No, for cherry-pick only

[vaxerski]

what advantages does this have over force_zero_scaling?

[moetayuko]

what advantages does this have over force_zero_scaling?

Quoting from my description in #6281:

The official Xwayland scaling solution xwayland:force_zero_scaling does not support multi-monitor with different scale factors. Specifically, when having two monitors of scale factors 2 and 1, Xwayland apps look good on the one of scale factor 2, but 2x larger on the one of scale factor 1.