Mushroom spread: Optimise and make overridable

Move mushroom spread ABM action into a global and overridable function.
Optimise spread code.
Reduce spread range to reduce spread through walls.

mods/flowers/init.lua

@@ -204,38 +204,34 @@ minetest.register_node("flowers:mushroom_brown", {

-- Mushroom spread and death

function flowers.mushroom_spread(pos, node)
	if minetest.get_node_light(pos, nil) == 15 then
		minetest.remove_node(pos)
		return
	end
	local positions = minetest.find_nodes_in_area_under_air(
		{x = pos.x - 1, y = pos.y - 2, z = pos.z - 1},
		{x = pos.x + 1, y = pos.y + 1, z = pos.z + 1},
		{"group:soil", "group:tree"})
	if #positions == 0 then
		return
	end
	local pos2 = positions[math.random(#positions)]
	pos2.y = pos2.y + 1
	if minetest.get_node_light(pos, 0.5) <= 3 and
			minetest.get_node_light(pos2, 0.5) <= 3 then
		minetest.set_node(pos2, {name = node.name})
	end
end

minetest.register_abm({
	label = "Mushroom spread",
	nodenames = {"flowers:mushroom_brown", "flowers:mushroom_red"},
	interval = 11,
	chance = 50,
	action = function(pos, node)
		if minetest.get_node_light(pos, nil) == 15 then
			minetest.remove_node(pos)
			return
		end
		local random = {
			x = pos.x + math.random(-2, 2),
			y = pos.y + math.random(-1, 1),
			z = pos.z + math.random(-2, 2)
		}
		local random_node = minetest.get_node_or_nil(random)
		if not random_node or random_node.name ~= "air" then
			return
		end
		local node_under = minetest.get_node_or_nil({x = random.x,
			y = random.y - 1, z = random.z})
		if not node_under then
			return
		end

		if (minetest.get_item_group(node_under.name, "soil") ~= 0 or
				minetest.get_item_group(node_under.name, "tree") ~= 0) and
				minetest.get_node_light(pos, 0.5) <= 3 and
				minetest.get_node_light(random, 0.5) <= 3 then
			minetest.set_node(random, {name = node.name})
		end
	end
	chance = 150,
	action = function(...)
		flowers.mushroom_spread(...)
	end,
})