Nix: add XWayland hidpi option

[fufexan]

Describe your PR, what does it fix/add?

Adds hidpiXWayland flag and the required logic to get it working.
Based on https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/733
The flag will be enabled by default, as I haven't had any issues with it in the past few days.

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

Users will have to adjust their config files accordingly, possibly having to add

exec-once=xprop -root -f _XWAYLAND_GLOBAL_OUTPUT_SCALE 32c -set _XWAYLAND_GLOBAL_OUTPUT_SCALE 2

in order for XWL to scale. This will make XWL programs look as if they were unscaled. To fix this, users have to export different environment variables to make the specific toolkits render at the proper scaling. For example

export GDK_SCALE=2
export XCURSOR_SIZE=32

The GDK_SCALE variable won't conflict with wayland-native GTK programs.

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

I'm still not sure whether we should go with this approach, or rather having a separate package.

@viperML @Misterio77 @nix guys for reviews

[viperML]

I would add another job to the CI action to build this

[viperML]

And also move the patching into its own wlroots.nix file

[fufexan]

@viperML I updated the HM module as well. Do you think I should also add a mkRenamedModule entry? Essentially it's xwayland -> xwayland.enable.

[viperML]

Sure. I don't even think anyone would use that option to disable xwayland

[fufexan]

Done. Mergeable?

[viperML]

Yes merge