engine: tab rendering code minor cleanups
engine: tab rendering code minor cleanups

Now the shape code also respects the x coordinate.

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