Screencasting is broken on:
xdg-desktop-portal-wlr-0.7.0-1.fc38.x86_64
wireplumber-0.4.14-1.fc38.x86_64
firefox-116.0-2.fc38.x86_64

I enabled verbosity on the xdg-desktop-portal-wlr service and using the https://mozilla.github.io/webrtc-landing/gum_test.html page in Firefox I attempt to start a screencast:

The following logs of the xdg-desktop-portal-wlr unit
```
Aug 10 09:26:59 fedora xdg-desktop-portal-wlr[7863]: 2023/08/10 09:26:59 [INFO] - xdpw: screencast instance 0x559ea434e280 has 1 references
Aug 10 09:26:59 fedora xdg-desktop-portal-wlr[7863]: 2023/08/10 09:26:59 [INFO] - xdpw: 1 active screencast instances
Aug 10 09:26:59 fedora xdg-desktop-portal-wlr[7863]: 2023/08/10 09:26:59 [INFO] - wlroots: output: eDP-1
Aug 10 09:26:59 fedora xdg-desktop-portal-wlr[7863]: 2023/08/10 09:26:59 [INFO] - dbus: session closed
Aug 10 09:26:59 fedora xdg-desktop-portal-wlr[7863]: 2023/08/10 09:26:59 [DEBUG] - dbus: destroying session 0x559ea434da50
Aug 10 09:26:59 fedora xdg-desktop-portal-wlr[7863]: 2023/08/10 09:26:59 [DEBUG] - xdpw: screencast instance 0x559ea434e280 now has 0 references
Aug 10 09:26:59 fedora xdg-desktop-portal-wlr[7863]: 2023/08/10 09:26:59 [DEBUG] - xdpw: destroying cast instance
```

The screnceast is created and destroyed immediately. There are no relevant logs in the firefox log either

Reproducible: Always

Steps to Reproduce:
1. Launch https://mozilla.github.io/webrtc-landing/gum_test.html
2. Select screen capture
3. Start screen capture
Actual Results:  
Screencapture starts and destroys immediatly