Color picker not working in Wayland

Hey there! The color picker in white balance (LMS) space is not working. I am not sure if this is a Wayland issue, but can categorically state that on my Wayland desktop it seems to be picking the background color of the desktop, and not the image that is being displayed in the project monitor.

I am doing a quick render now, and will test the color picker on X11 as well. It's a bit of a pain to switch between X11 and Wayland, as they use different methods for scaling.

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Edit @emohr

Documentation for screenshotting: https://flatpak.github.io/xdg-desktop-portal/#gdbus-org.freedesktop.portal.Screenshot (see #1417 (comment 509261))