Default: Add 'acacia leaves simple' texture

A deeper, darker level of leaves is created by tiling the texture
2 by 2, reducing brightness and offsetting this.
For a denser leaf appearence with the 'simple leaves' setting.
Also used for acacia bush leaves.

mods/default/README.txt

@@ -88,6 +88,7 @@ paramat (CC BY-SA 3.0):
   default_pinetree_top.png
   default_pinewood.png
   default_acacia_leaves.png
+  default_acacia_leaves_simple.png
   default_acacia_sapling.png
   default_acacia_tree.png
   default_acacia_tree_top.png

mods/default/nodes.lua

@@ -790,6 +790,7 @@ minetest.register_node("default:acacia_leaves", {
 	description = "Acacia Leaves",
 	drawtype = "allfaces_optional",
 	tiles = {"default_acacia_leaves.png"},
+	special_tiles = {"default_acacia_leaves_simple.png"},
 	waving = 1,
 	paramtype = "light",
 	is_ground_content = false,
@@ -1264,7 +1265,7 @@ minetest.register_node("default:acacia_bush_leaves", {
 	description = "Acacia Bush Leaves",
 	drawtype = "allfaces_optional",
 	waving = 1,
-	tiles = {"default_acacia_leaves.png"},
+	tiles = {"default_acacia_leaves_simple.png"},
 	paramtype = "light",
 	groups = {snappy = 3, flammable = 2, leaves = 1},
 	sounds = default.node_sound_leaves_defaults(),