Support Qt scaling in wallpaper loading

Summary:
On wayland we're using Qt scaling.
This normally handles most scaling stuff itself, except because we're
explicitly forcing a source size, we need to scale that.

Test Plan:
Loaded a 4k wallpaper from internet
Looked at the tiny pixels

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8826