notebook: fix scaling of rendered background
notebook: fix scaling of rendered background

The background was rendered with the wrong height, which made the
linear gradient end abruptly.

Spotted by Jack Gandy <scionicspectre@gmail.com>
https://bugzilla.gnome.org/show_bug.cgi?id=672280

rgcjonas/adwaita-semidark,magcius/gnome-themes-standard,ChrisCummins/gnome-themes-standard,cosimoc/gnome-themes-standard,ChrisCummins/gnome-themes-standard,ChrisCummins/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,Distrotech/gnome-themes-standard,magcius/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,magcius/gnome-themes-standard,Distrotech/gnome-themes-standard,ChrisCummins/gnome-themes-standard,rgcjonas/adwaita-semidark,ChrisCummins/gnome-themes-standard,cosimoc/gnome-themes-standard,rgcjonas/adwaita-semidark,cosimoc/gnome-themes-standard,rgcjonas/adwaita-semidark,cosimoc/gnome-themes-standard