thumbnailer: Cleanup the cleanups
thumbnailer: Cleanup the cleanups

Commit 6afda9c removed the separate height and width arguments, as we
always want to keep the original image's aspect ratio. The cleanup
wasn't quite finished though, and ended up with slightly non-sensical
checks (like doing comparisons twice).

Coverity CID 1388524

https://bugzilla.gnome.org/show_bug.cgi?id=768062

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf