Improve mapgen, add grass, and include the fill tool.

mods/rgt_world/mapgen.lua

@@ -0,0 +1,14 @@
+
+local c = minetest.get_content_id
+
+local c_stone = c "stone"
+
+local np_continental = {
+    offset = 0,
+    scale = 1,
+    spread = {x = 4096, y = 4096, z = 4096},
+    seed = 12345,
+    octaves = 4,
+    persist = 0.6
+}
+local n_terrain = {}