ScreenCast: Fix VIRTUAL source type typo
ScreenCast: Fix VIRTUAL source type typo

It's a bit mask, and 3 is just 2|1. It should have been 4, which was
also what was implemented.

matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal