perf: fix wallpaper style for overridden background

The code that loads SHELL_BACKGROUND_IMAGE, which is used to load the
performance background was loading it in WALLPAPER mode, not ZOOM
mode. Zoom mode is what we use for the actual GNOME defaultiwallpaper
and what we want to test: the background will be scaled except when
the resolution matches the 2560x1440 default backgrounds.

https://bugzilla.gnome.org/show_bug.cgi?id=735385