Pipewire + camera support

I'm currently trying to get rid of `--device=all` in my flatpak manifest. Doing this temporarily with flatseal / [org.chromium.Chromium](https://github.com/flathub/org.chromium.Chromium) leads to a portal query if I want to allow camera access. This works with without V4L, just pipewire.

Removing the `--device=all` in my own app, doesn't seem work, although the required flatpak patches for qtwebengine seem to be in place.

I've `--enable-features=WebRtcPipeWireCamera` in the **QTWEBENGINE_CHROMIUM_FLAGS**, but there's no portal query - not even something on DBus that tries to speak with the flatpak portal. What I see is

```
Failed to connect to pipewire instance "Host is down"
```

, though. So it seems the process responsible for talking with pipewire is unable to do so. There is no pipewire socket, because this should be handled by the portal. The Camera Portal is available in the sandbox:

```
$ dbus-send --session --dest=org.freedesktop.portal.Desktop \
--print-reply /org/freedesktop/portal/desktop \
org.freedesktop.DBus.Properties.Get \
string:"org.freedesktop.portal.Camera" string:"IsCameraPresent"
method return time=1773654712.078706 sender=:1.72 -> destination=:1.375 serial=2657 reply_serial=2
variant boolean true
```

so the "Host is down" seems to indicate that it takes the socket path instead of the portal path for whatever reason. According to the qtwebengine build logs and the manifest, the rtc/pipewire path *should* be enabled. Hmm.

I tried other flatpaks like i.e. Falkon, but there are also no portal dialogs when accessing the camera. Is this an issue with the patches, or am I doing something wrong?