Improve map background

[Tianjian]

When there is nothing in the background, we could barely know if we are moving, due to the lack of reference.

At the early stage I would suggest to add grid to the background

[Théo Pannetier]

Yeaaaah I worked on this a long while ago, the horrendous red/blue/green patterns on the map are because at some point we wanted to offer players a chance to camouflage on the background (I was successful in that respect). I agree that it is disruptive to orientation; would be great if someone could suggest / implement an alternative 😁