getDimensionId() returns the dimension id - 1. So without this patch
we would reuse an existing dimension id, if some other dimension was
unloaded before.
In Spigot this is nearly invisible because DimensionManager has no equals(),
so dimension id collisions just create 2 worlds with the same dimension.
The PaperWorldMap (Added in
https://github.com/PaperMC/Paper/blob/master/Spigot-Server-Patches/0376-Optimize-Server-World-Map.patch ) changes this - Now the dimension is overwritten if there is some collision,
what causes players to teleport to incorrect worlds, World checks will no longer work
and many more evil things.
