add cobblestone and fix some drops.

add cobblestone node. make stone drop cobblestone and grass drop dirt.

mods/bg_core/node.lua

@@ -1,6 +1,17 @@
 local modname = minetest.get_current_modname()
 
-blockgame.reg_simple_node("stone", "Stone", {
+minetest.register_node(modname .. ":stone", {
+	description = "Stone",
+	tiles = {
+		modname .. "_stone.png",
+	},
+	groups = {
+		cracky = 3,
+		stoney = 1,
+	},
+	drop = modname .. ":cobblestone",
+})
+blockgame.reg_simple_node("cobblestone", "Cobblestone", {
 	cracky = 3,
 	stoney = 1,
 })
@@ -8,11 +19,17 @@ blockgame.reg_simple_node("stone", "Stone", {
 minetest.register_alias("mapgen_stone", modname .. ":stone")
 minetest.register_alias("mapgen_water_source", "air")
 
-blockgame.reg_simple_node("grass", "Grass", {
+minetest.register_node(modname .. ":grass", {
+	description = "Grass",
+	tiles = {
+		modname .. "_grass.png",
+	},
+	groups = {
 		cracky = 3,
 		dirty = 1,
+	},
+	drop = modname .. ":dirt",
 })
-
 blockgame.reg_simple_node("dirt", "Dirt", {
 	cracky = 3,
 	dirty = 1,