Plasma Wallpaper Image Plugin: Avoid unnecessary renders of the wallpaper

When the selection in changed in the view, that calls Image::pictureChanged
which then calls setSingleImage which sends the image for rendering.
Once the image has been rendered, updateBackground is called which
starts the fading animation. This is completely unnessary.
(cherry picked from commit 3ae865333c40b9fb4de09d2c0a6cf00ec50a0382)