GDK_SUPER_MASK is not set on Super key press events on Wayland / wlroots

When handling a key press event for something like "Super+/", the GDK_SUPER_MASK bit is not set on Sway; only the GDK_MOD4_MASK bit is. This is different from the documented behavior, which I could reproduce using X11 with Xwayland.

Steps to reproduce

Download the following script: https://paste.sr.ht/~schnouki/9f76166c5287de6a1e7d3d3f1537fe2fbd511d5e (sorry, didn't try to do it in C; if needed please let me know and I'll do what I can)
Run it under Sway, hit "Super+/": on the console, the following output appears:
```
$ ./gtk-events.py
event.keyval=65515 event.string='' event.state=<flags 0 of type Gdk.ModifierType>
event.keyval=47 event.string='/' event.state=<flags GDK_MOD4_MASK of type Gdk.ModifierType>
```
First event is the Super_L key on its own, 2nd one is Super+/. There's GDK_MOD4_MASK in the modifiers, but no GDK_SUPER_MASK.

Run the same script under Xwayland and do the same thing:

$ WAYLAND_DISPLAY="" XDG_SEAT="" ./gtk-events.py
event.keyval=65515 event.string='' event.state=<flags 0 of type Gdk.ModifierType>
event.keyval=47 event.string='/' event.state=<flags GDK_MOD4_MASK | GDK_SUPER_MASK of type Gdk.ModifierType>

This time the GDK_SUPER_MASK is set.

Current behavior

The GDK_SUPER_MASK bit is not set on key press events when Super is pressed, only GDK_MOD4_MASK is.

Expected outcome

The GDK_SUPER_MASK bit should be set too.

Version information

GTK 3.24.33 (from the Arch repo)
Arch Linux
Sway 1.7, wlroots 0.15.1, xorg-xwayland 22.1.1

Additional information

This was found while trying to understand a bug in recent Git versions of Emacs: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=55362. It used to work because Emacs didn't rely on GDK modifier flags and re-implemented a similar mechanism, which was recently removed.