Only shows the desktop without windows on macOS Catalina

[KaiHofstetter]

Starting with Catalina Apple introduced the "screen recording" permission, which is needed to create screenshots of other apps.

It seems that CGWindowListCopyWindowInfo and CGWindowListCreateImage don't work anymore.

More info from Apple can be found (here)[https://developer.apple.com/videos/play/wwdc2019/701/]

On regular apps those calls would trigger a popup window, which asks the user for permission. But this doesn't seem the case for screensavers.