L-Systems fix randomness

Unless a random seed is provided (via Lua treedef) seed the PRNG with a different seed for each tree
Resolves: https://github.com/minetest/minetest/issues/1469

doc/lua_api.txt

@@ -2130,7 +2130,7 @@ treedef={
   thin_branches, - boolean true -> use thin (1 node) branches
   fruit,         - string  fruit node name
   fruit_chance,  - num     chance (0-100) to replace leaves with fruit node
-  seed,          - num     random seed
+  seed,          - num     random seed; if no seed is provided, the engine will create one
   }
 
 Key for Special L-System Symbols used in Axioms

src/script/lua_api/l_env.cpp

@@ -757,7 +757,7 @@ int ModApiEnvMod::l_spawn_tree(lua_State *L)
 			tree_def.fruitnode=ndef->getId(fruit);
 			getintfield(L, 2, "fruit_chance",tree_def.fruit_chance);
 		}
-		getintfield(L, 2, "seed", tree_def.seed);
+		tree_def.explicit_seed = getintfield(L, 2, "seed", tree_def.seed);
 	}
 	else
 		return 0;

src/treegen.cpp

@@ -149,8 +149,17 @@ void make_ltree(ManualMapVoxelManipulator &vmanip, v3s16 p0, INodeDefManager *nd
 		TreeDef tree_definition)
 {
 	MapNode dirtnode(ndef->getId("mapgen_dirt"));
+	int seed;
+	if (tree_definition.explicit_seed)
+	{
+		seed = tree_definition.seed+14002;
+	}
+	else
+	{
+		seed = p0.X*2 + p0.Y*4 + p0.Z;      // use the tree position to seed PRNG
+	}
+	PseudoRandom ps(seed);
 
-	PseudoRandom ps(tree_definition.seed+14002);
 	// chance of inserting abcd rules
 	double prop_a = 9;
 	double prop_b = 8;

src/treegen.h

@@ -50,6 +50,7 @@ namespace treegen {
 		MapNode fruitnode;
 		int fruit_chance;
 		int seed;
+		bool explicit_seed;
 	};
 
 	// Add default tree