Move Meta.WindowType.DESKTOP to background in WorkspaceAnimation
Currently if there is a window that is on_all_workspaces, it is considered sticky and sticks to the monitor while all animations happens behind it on the background.
This is a problem for desktop windows, which are on_all_workspaces, but should be rendered behind all windows in the animation.
This MR detects a Meta.WindowType.DESKTOP that is on_all_windows, makes a clone and puts in in the background for the animation for that workspace. The rest of the windowss should work in the same way.