Lua voxelmanip: Add optional buffer param for 'get param2 data'

Update lua_api.txt.

doc/lua_api.txt

@@ -3263,7 +3263,9 @@ will place the schematic inside of the VoxelManip.
 * `set_light_data(light_data)`: Sets the `param1` (light) contents of each node
   in the `VoxelManip`
     * expects lighting data in the same format that `get_light_data()` returns
-* `get_param2_data()`: Gets the raw `param2` data read into the `VoxelManip` object
+* `get_param2_data([buffer])`: Gets the raw `param2` data read into the `VoxelManip` object
+    * Returns an array (indices 1 to volume) of integers ranging from `0` to `255`
+    * If the param `buffer` is present, this table will be used to store the result instead
 * `set_param2_data(param2_data)`: Sets the `param2` contents of each node in the `VoxelManip`
 * `calc_lighting([p1, p2], [propagate_shadow])`:  Calculate lighting within the `VoxelManip`
     * To be used only by a `VoxelManip` object from `minetest.get_mapgen_object`

src/script/lua_api/l_vmanip.cpp

@@ -277,11 +277,17 @@ int LuaVoxelManip::l_get_param2_data(lua_State *L)
 	NO_MAP_LOCK_REQUIRED;
 
 	LuaVoxelManip *o = checkobject(L, 1);
+	bool use_buffer  = lua_istable(L, 2);
+
 	MMVManip *vm = o->vm;
 
 	u32 volume = vm->m_area.getVolume();
 
-	lua_newtable(L);
+	if (use_buffer)
+		lua_pushvalue(L, 2);
+	else
+		lua_newtable(L);
+
 	for (u32 i = 0; i != volume; i++) {
 		lua_Integer param2 = vm->m_data[i].param2;
 		lua_pushinteger(L, param2);