Flora/tree generation doesn't work on chunk border

[Unarelith]

Let's say we have two chunks A and B and A is on top of B.

If B loads first, A will be able to know what blocks are in the top of B to spawn (or not) trees and flora.
If A loads first, it won't be able to know B blocks, so it won't generate trees and flora.

A potential solution for this problem would be to process flora/tree generation when placing the top block, instead of looking for a top block while in the air.

The problem was here since a long time, but is now very visible since chunks are cubic.