It would be great if Xwayland clients could render at the native
resultion, at least in fullscreen, when (fractional) scaling is
used in the Wayland compositor.
We can archive that by using the existing Xrandr mode emulation,
based on wp_viewporter, in a reversed way. As the scaling happens
on the compositor level (possibly even in hardware planes), we
only have to ensure a 1-1 mapping of the backing buffer with the
native screen resolution, while the viewport destination size
matches the logical resolution.
It discovers the native resolution using wl_output and
allows me to run games such as Xonotic to be sharply rendered in
native resolution (tested with 1920x1080) in a Gnome session
with 125% scaling. The correct values are also detected on KDE
and Sway.