Fix map overlay shift

[Sinclert]

There exist a certain shift on the map tiles when zoom reaches value 4.5 (18 zoom increases, at 0.25 increase each). This shift makes previous coordinate system and rendered figures to suffer a North-West displacement (which seems to be constant across the map).

Further research must determine if this is a bug coming from PaperScape tiles, or a problem with React Leaflet library.