Biome API: Add 'get_biome_name(biome_id)' API

Change name of default biome to a more suitable lowercase 'default'.

doc/lua_api.txt

@@ -2959,6 +2959,10 @@ handler.
 * `minetest.get_biome_id(biome_name)`
     * Returns the biome id, as used in the biomemap Mapgen object and returned
       by `minetest.get_biome_data(pos)`, for a given biome_name string.
+* `minetest.get_biome_name(biome_id)`
+    * Returns the biome name string for the provided biome id, or `nil` on
+      failure.
+    * If no biomes have been registered, such as in mgv6, returns `default`.
 * `minetest.get_mapgen_params()`
     * Deprecated: use `minetest.get_mapgen_setting(name)` instead.
     * Returns a table containing:

src/mapgen/mg_biome.cpp

@@ -40,7 +40,7 @@ BiomeManager::BiomeManager(Server *server) :
 	// Create default biome to be used in case none exist
 	Biome *b = new Biome;
 
-	b->name            = "Default";
+	b->name            = "default";
 	b->flags           = 0;
 	b->depth_top       = 0;
 	b->depth_filler    = -MAX_MAP_GENERATION_LIMIT;

src/script/lua_api/l_mapgen.cpp

@@ -476,12 +476,10 @@ int ModApiMapgen::l_get_biome_id(lua_State *L)
 		return 0;
 
 	BiomeManager *bmgr = getServer(L)->getEmergeManager()->biomemgr;
-
 	if (!bmgr)
 		return 0;
 
 	Biome *biome = (Biome *)bmgr->getByName(biome_str);
-
 	if (!biome || biome->index == OBJDEF_INVALID_INDEX)
 		return 0;
 
@@ -491,6 +489,25 @@ int ModApiMapgen::l_get_biome_id(lua_State *L)
 }
 
 
+// get_biome_name(biome_id)
+// returns the biome name string
+int ModApiMapgen::l_get_biome_name(lua_State *L)
+{
+	NO_MAP_LOCK_REQUIRED;
+
+	int biome_id = luaL_checkinteger(L, 1);
+
+	BiomeManager *bmgr = getServer(L)->getEmergeManager()->biomemgr;
+	if (!bmgr)
+		return 0;
+
+	Biome *b = (Biome *)bmgr->getRaw(biome_id);
+	lua_pushstring(L, b->name.c_str());
+
+	return 1;
+}
+
+
 // get_heat(pos)
 // returns the heat at the position
 int ModApiMapgen::l_get_heat(lua_State *L)
@@ -1731,6 +1748,7 @@ int ModApiMapgen::l_serialize_schematic(lua_State *L)
 void ModApiMapgen::Initialize(lua_State *L, int top)
 {
 	API_FCT(get_biome_id);
+	API_FCT(get_biome_name);
 	API_FCT(get_heat);
 	API_FCT(get_humidity);
 	API_FCT(get_biome_data);

src/script/lua_api/l_mapgen.h

@@ -28,6 +28,10 @@ class ModApiMapgen : public ModApiBase
 	// returns the biome id as used in biomemap and returned by 'get_biome_data()'
 	static int l_get_biome_id(lua_State *L);
 
+	// get_biome_name(biome_id)
+	// returns the biome name string
+	static int l_get_biome_name(lua_State *L);
+
 	// get_heat(pos)
 	// returns the heat at the position
 	static int l_get_heat(lua_State *L);