Minetest: Mgv7: Stone in tunnels in mountain terrain at y = 48 when generating downwards

[paramat]

I know the cause, mountain terrain is allowed to generate in air by necessity, so it generates stone in tunnels at node_max.Y + 1 when the lower mapchunk generates after the upper.
It can be fixed by re-allowing tunnel generation to overgenerate 1 up and 1 down, but then we lose the 'roof' trick bb2ee54d3b841ebdb6c6a0b6573b64216ae0baae that stops light in tunnels at mapchunk borders.
I will see what i can do.

[paramat]

See #4002

[est31]

Issue 4k celebrate! :tada:

[paramat]

4002 is merged.