Make `World->getSafeSpawn()` more intelligent

[dktapps]

Problem description

World->getSafeSpawn() will currently not consider a 2-block space between two slabs like so:

Top half slab
Air
Bottom half slab

Back in the day, this enabled the creation of magic respawn elevators, where a player would stand on top of a lower slab with an upper slab in the next block up but one, which would cause the server to respawn the player in a higher position. I used this to escape caves while lost.

Proposed solution

World->getSafeSpawn() should consider block collision boxes and determine whether there is at least 1.8m of vertical space available at a chosen position, rather than searching for 2 air blocks.

Alternative solutions or workarounds

No response