Make the logout screen's background just be opaque black

Reason for the change

The desired UX here is to have some content shown above a big black overlay. Right now the overlay is very slightly transparent, allowing the blurred background that KWin provides to show through. But the amount of transparency is not enough for this to be clear to the user, so the visual impact ends up practically imperceptible.

In addition, there is some code to sometimes change the background color to that of the active color scheme, which appears to not work properly (which is good, because if it did work, it could cause readability issues with certain color schemes).

Simplify this by just showing a flat black opaque background. This is what the UI elements are expecting anyway because they are hardcoded to a white color.

@teams/vdg

Test plan

Run ~/kde/usr/lib64/libexec/ksmserver-logout-greeter --windowed

Screenshots or screen recordings

Before	After

Here the "before" picture depicts the logout screen shown immediately above my colorful wallpaper, which is absolute best-case scenario for anything below it to be visible.