notebook: don't trim the allocated area when drawing the tab shape
notebook: don't trim the allocated area when drawing the tab shape

This causes drawing artifacts and is probably here because of a previous
bug in GtkNotebook preventing the tab shape to render cleanly.

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