Add underground tiles to forest worldmap

I noticed this in SuperTux 0.6.0-rc.1, and it's still preset currently, in SuperTux_2-v0.6.0-36-g9cb55e3e2.

The level 'Through the Dark' is hidden in a tunnel (I guess), but you can't see where you are.

I should add that the same issue occurs by the next level, 'What is happening?'.

[Rusty-Box]

While originally intented, since creating a visual underground section for just one level felt a bit unnecessary, new underground forest tiles for worldmap building are on their way. This of course also means that the forest worldmap will get another update fixing this issue.

[tobbi]

@Rusty-Box Is this fixed?

I just tested the latest nightly (SuperTux_2-v0.6.2-933-g5e5e1378a.glibc2.29-x86_64) and it is not yet fixed.

[komaros1]

uhh why is this still open? there is going to be a new worldmap for the forest (at least i think)

[Hypernoot]

Let this issue be a reminder that the forest worldmap shall be redesigned. It will be resolved by the forest world PR when it's done.