[gtk3/wayland] Fix GdkMonitor sizes on Mutter without fractional scaling.

This is the gtk3 version of !6722 (closed).

This fixes a problem that is apparent in https://bugzilla.mozilla.org/show_bug.cgi?id=1869724, but that also reproduces on any GTK application as described in https://bugzilla.mozilla.org/show_bug.cgi?id=1869724#c16.

xdg_output sizes might be physical if the compositor doesn't scale them, it seems. So to report the correct logical geometry in GDK pixels, we need to detect this case. We do this by checking whether the wl_output size matches the xdg_output size.