When 4 spruce saplings are put into a 2×2 square, they may grow into a
large spruce tree. With this patch, dirt group group nodes nearby that
have air above and nodes under the stem are replaced by podzol nodes.
Note that this commit uses a different placement method than MineClone2
commit a83a2e9aba122d0fb4f1c120252a0d4eb07a1f10 to create one connected
podzol patch instead of randomly scattered nodes. It also places podzol
nodes in bulk at once for better performance.
6059b79785