Harrier-GR3: in 'Harrier' menu, add item to toggle terrain map.
This helps framerate, e.g. from 23 to 28fps.
Disables if property /sim/terrain-map is 0; this property is saved to aircraft-data so will persist between sessions.
Authored by: cgdae 2020-03-28
Parent: [r4686]
Child: [r4688]
