make some leaves non-walkable.

make leaves and decomposing_leaves non-walkable, and make them support
falling nodes.

mods/bg_tree/node_leaves.lua

@@ -44,6 +44,8 @@ local nutty_drops = {
 
 reg_leaves("leaves", {
 	description = "Leaves",
+	walkable = false,
+	supports_falling = true,
 })
 reg_leaves("leaves_growing", {
 	description = "Growing Leaves",
@@ -114,6 +116,8 @@ local function reg_decomposing (level)
 		groups = {
 			leaves_decomposing = level,
 		},
+		walkable = false,
+		supports_falling = true,
 	})
 end
 for level=1, decomposing_leaves_layers do