Use the native resolution image

Summary:
Instead of loading the image size of the scaled window, use the actual resolution of the display.
This way no scaling will be needed on high dpi displays.

Test Plan: Added debug statements that load the wallpaper that matches my monitor size.

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22078