Bug 494630

SUMMARY
I use Wayland and KWin on my SDDM. When I updated to 6.2.0, the login screen and the splash screen of Plasma becomes quite dark, and goes back to normal after Plasma is fully loaded. I have tried `cat /sys/class/backlight/nvidia_wmi_ec_backlight/brightness` on a tty, which shows that backlight brightness is the same as normal (25% brightness). It seems that apart from managing the backlight, KWin is reducing the brightness of the pixels. My current workout is editing `/var/lib/sddm/.config/kwinoutputconfig.json` and set `brightness` to `1`, but the splash screen is still dimmed.

STEPS TO REPRODUCE
Happens on its own and I have no idea. Will provide further information if any test required.

OBSERVED RESULT
Using KWin outside Plasma gets brightness dim.

EXPECTED RESULT
Screen dim is controlled by backlight brightness as before.

SOFTWARE/OS VERSIONS
KWin Version: 6.2.0
KDE Plasma Version: 6.2.0
KDE Frameworks Version: 6.6.0
Qt Version: 6.7.3
Kernel Version: 6.11.3-zen1-1-zen (x64)

ADDITIONAL INFORMATION
SDDM Wayland configuration:
```
[General]
DisplayServer=wayland
GreeterEnvironment=QT_WAYLAND_SHELL_INTEGRATION=layer-shell

[Wayland]
CompositorCommand=kwin_wayland --drm --no-lockscreen --no-global-shortcuts --locale1
```