routed spawn_tree() through a wrapper function so that other mods can hook into

plants_lib's calls to do something extra when a tree is generated at mapgen
time or spawned via the growing code.

API.txt

@@ -22,6 +22,12 @@ All functions in plants lib are declared locally to avoid namespace collisions
 with other mods.  They are accessible via the "plantslib" method, e.g.
 plantslib:spawn_on_surfaces() and so forth.
 
+In the case of the growing code and the mapgen-based tree generator code, the
+usual spawn_tree() calls used therein are routed through
+plantslib:generate_tree() and plantslib:grow_tree(), which just directly call
+the spawn_tree() function anyway.  This way other mods can wedge into these 
+two calls by name, to execute something extra whenever a tree is spawned via
+this library.
 
 =====
 spawn_on_surfaces(biome)

plants_lib/init.lua

@@ -133,7 +133,7 @@ function plantslib:search_for_surfaces(minp, maxp, biomedef, node_or_function_or
 							end
 							]]--
 
-							minetest.env:spawn_tree(pos, node_or_function_or_model)
+							plantslib:generate_tree(pos, node_or_function_or_model)
 
 						elseif type(node_or_function_or_model) == "string" then
 							if minetest.registered_nodes[node_or_function_or_model] == nil then
@@ -353,7 +353,7 @@ function plantslib:replace_object(pos, replacement, grow_function, walldir, seed
 	if growtype == "table" then
 		plantslib:dbg("Grow: spawn tree at "..dump(pos))
 		minetest.env:remove_node(pos)
-		minetest.env:spawn_tree(pos, grow_function)
+		plantslib:grow_tree(pos, grow_function)
 		return
 	elseif growtype == "string" then
 		local perlin1 = minetest.env:get_perlin(seeddiff, perlin_octaves, perlin_persistence, perlin_scale)
@@ -418,4 +418,17 @@ function plantslib:find_open_side(pos)
 	return nil
 end
 
+-- spawn_tree() on generate is routed through here so that other mods can hook
+-- into it.
+
+function plantslib:generate_tree(pos, node_or_function_or_model)
+	minetest.env:spawn_tree(pos, node_or_function_or_model)
+end
+
+-- and this one's for the call used in the growing code
+
+function plantslib:grow_tree(pos, node_or_function_or_model)
+	minetest.env:spawn_tree(pos, node_or_function_or_model)
+end
+
 print("[Plantlife Library] Loaded")