wallpapers/image: Use QImageReader in ImageSizeFinder::run()

Authored by fredrik on Dec 21 2016, 12:46 PM.

Description

wallpapers/image: Use QImageReader in ImageSizeFinder::run()

There is no need to decode the whole image and load it into memory
when we are only interested in the dimensions.

Differential Revision: https://phabricator.kde.org/D3973