Blank window on startup unless --disable-gpu is specified

I just switched to Fedora Silverblue 35 and want to avoid installing software on the host OS if possible.

Running this flatpak resulted in a *blank* window frame with a correct title but only grey contents.
As my main desktop is GNOME on Wayland, I tried the instructions in PR #229 which kind-of worked but as stated there, this disables the window frame so I have only the correct contents without any way to maximize etc.
Google was of no help but by accident I stumbled upon the `--disable-gpu` flag, and it turns out that this fixes my problem.

GPU info for this machine:

```
$ lspci -v -s $(lspci | grep ' VGA ' | cut -d" " -f 1)
01:00.0 VGA compatible controller: NVIDIA Corporation GM200 [GeForce GTX 980 Ti] (rev a1) (prog-if 00 [VGA controller])
Subsystem: ASUSTeK Computer Inc. Device 8548
Flags: bus master, fast devsel, latency 0, IRQ 134
Memory at f6000000 (32-bit, non-prefetchable) [size=16M]
Memory at e0000000 (64-bit, prefetchable) [size=256M]
Memory at f0000000 (64-bit, prefetchable) [size=32M]
I/O ports at e000 [size=128]
Expansion ROM at 000c0000 [disabled] [size=128K]
Capabilities:
Kernel driver in use: nouveau
Kernel modules: nouveau
```

Most notably I'm *not* using the proprietary nvidia driver:

```
$ lsmod | grep '^drm'
drm_kms_helper 311296 1 nouveau
drm_ttm_helper 16384 1 nouveau
drm 630784 12 drm_kms_helper,drm_ttm_helper,ttm,nouveau
```

Would like to understand if the flag can be dropped in some way.

Es werden keine Repository-Dateien oder Tests genannt. Beginne damit, das leere Fenster auf Fedora Silverblue 35 mit GNOME unter Wayland und Nouveau zu reproduzieren, und vergleiche anschließend den Start mit --disable-gpu und den Anweisungen aus PR #229; als erledigt gilt die Identifizierung einer unterstützten Lösung, die den Fensterrahmen erhält, oder die Dokumentation, warum der Workaround erforderlich ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.