wallpapers/image: fix side-by-side previews sometimes not showing

QCache will remove a cached item when it reaches the maximum allowed
total cost of the cache, so it's not stable to use QCache as a transfer
station to store side-by-side previews that are still under
construction, or there will only be dark previews for wallpapers with
both light and dark variants.


(cherry picked from commit 7a618142)