Don't fade wallpapers after resizing

When the wallpaper "changes" because of screen resolution change or the
screen was rotated, the fade animation of the wallpaper actually makes
it look more glitchy, making visible for a while an image scaled to the
wrong resolution.

Things like autorotation now looks much smoother

before:

![VID_20231002_165454](/uploads/106eff8c938b91148dabae66a2e87ace/VID_20231002_165454.mp4)

after:

![VID_20231002_170610](/uploads/deda7dea7ea738bc2aa2c1a05fa4f64c/VID_20231002_170610.mp4)

see also kdeplasma-addons!472