wallpapers/potd: Fix image opacity on first load

Otherwise, when the replace method is called with the Immediate operation, the opacity is set by the transition to 0 and never updated.

This fixes the wallpaper not showing when plasmashell is started.

The cross-fade transition is still preserved when changing the wallpaper later.

This doesn't happen on Plasma 5, and this code has barely changed since, so it might be a Qt bug on StackView transitions?