Level heights should be separate from top and bottom layer distances

[haykam821]

The following command opens a game with a level height of 1:

game open {"type":"spleef:spleef",players:{min:1,max:100},map:{"shape": { "type": "spleef:circle", "radius": 16 }, "levels": 4, "level_height": 1}}

However, the generated map has a ceiling directly above the top layer:

Being able to separately configure the level height from the ceiling would allow greater flexibility in smaller level heights.