Fix background scaling on multi-monitor setups

https://bugzilla.gnome.org/show_bug.cgi?id=648193



apply_background_to_window() in gs-manager.c uses the screen width and height to calculate the background size, which causes the background to be scaled incorrectly in multi-monitor setups.

This fixes the calculation of the background size. However, spanned backgrounds require additional work.

Signed-off-by: Martin Schmölzer <martin.schmoelzer@student.tuwien.ac.at>