Black desktop on startup/during initial load

Issue Description

During login/startup there's a black background/desktop in place of wallpaper, until it gets loaded and displayed.
Appeared at, and since 4.19.5 (older 4.19.x versions fail to build for me), Wasn't the case with 4.18.

Main issue is a somewhat jarring sequence of: (lightdm) wallpaper -> black desktop until bg image loads -> wallpaper again (I use the same image for both lightdm and desktop in which case with 4.18 the switch from lightdm to xfdesktop was completely invisible, not even a flicker).

There is no black background when changing wallpapers from settings, previous one is retained on screen until new one is loaded.

Split from #347 (closed).

Steps to Reproduce Issue

1. Observe behavior (black desktop/wallpaper) during login

Expected Behavior

1. Retain what was previously on screen until wallpaper is loaded (instead of blacking it out)

Additional Information

Will provide any further info as needed/requested.

Environment & Versions

• System:
  • Phenom II X6 @3.8GHz, 16GB RAM, Radeon RX570
• OS distribution and version: Debian 12
• Overall Xfce version: 4.20pre1 through 4.20.0
• xfdesktop version (git hash if not a release): 4.19.5 through 4.20.0
• Windowing system (X11/Wayland): X11
• Compositor and version (if Wayland): xfwm 4.19.0-4.20.0
• GTK Theme: Custom Matcha
• Icon Theme: Vibrancy Color Dark
• UI Scale Factor: 1x (130DPI)