xfce-bg: Fix wallpaper issues with Xfce 4.20
Found when testing !49 (merged). This fixes "I have a fresh Xfce 4.20 setup but cannot see wallpaper in screensaver before I changed both my wallpaper and image-style settings".
I wrote my findings in the commit messages, I will specifically thank GitLab search for allowing me to search xfce_workspace_migrate_backdrop_image_style
on 4.18 branch
Here is a fresh installed Xfce 4.18 VM and a fresh installed Xfce 4.20 VM:
Edited by Bobby Rong