Add a simple, but stylish fence gate.

This fence gate builds on NDT_CONNECTED by assuming fence nodes will
automatically connect to it's sides properly. The fence gate will
open and close just like doors, with sounds, but it only opens one
way. The gate sticks out quite a bit and can be bumped into, so the
fence may be used as some sort of path switch.

The fence gate offers no form of protection and can be opened and
closed by anyone. This is done on purpose - the fencegate isn't
meant to provide protection from players, as fences can be
trivially jumped over. Instead, these fences should be used for
protecting crops from hungry sheep, or keeping rabbits in their
pen, or just decoration. Mods can also modify the mod to add
protection, of course.

A recipe is added to make these. It's 4 sticks and 2 wood (any)
as follows:

  stick  wood  stick
  stick  wood  stick

The collision box of the open gate is such that if two gates are
connected but mirrored (making an M shape) then you can walk a large
entity that's larger than 1.0 wide through the opening. The gate of
an opened fence can also be stood upon or bumped into.

I've mixed together some sounds to provide a somewhat light sound
experience, one that one would expect from a small gate latching open
and close.

This change requires #873, otherwise it doesn't connect to fences.

mods/doors/README.txt

@@ -7,9 +7,10 @@ License of source code:
 Copyright (C) 2012 PilzAdam
 modified by BlockMen (added sounds, glassdoors[glass, obsidian glass], trapdoor)
 Steel trapdoor added by sofar.
-Copyright (C) 2015 sofar@foo-projects.org
+Copyright (C) 2016 sofar@foo-projects.org
 Re-implemented most of the door algorithms, added meshes, UV wrapped texture
 Added doors API to facilitate coding mods accessing and operating doors.
+Added Fence Gate model, code, and sounds
 
 This program is free software. It comes without any warranty, to
 the extent permitted by applicable law. You can redistribute it
@@ -43,9 +44,6 @@ following textures created by sofar (CC-BY-SA-3.0)
   doors_trapdoor_steel_side.png
   door_trapdoor_side.png
 
-Door 3d models by sofar (CC-BY-SA-3.0)
-  door_a.obj
-  door_b.obj
 
 Obsidian door textures by red-001 based on textures by Pilzadam and BlockMen: WTFPL
   door_obsidian_glass.png
@@ -57,6 +55,14 @@ All other textures (created by PilzAdam): WTFPL
 Door textures were converted to the new texture map by sofar, paramat and
 red-001, under the same license as the originals.
 
+Models:
+--------------------------------------
+Door 3d models by sofar (CC-BY-SA-3.0)
+ - door_a.obj
+ - door_b.obj
+Fence gate models by sofar (CC-BY-SA-3.0)
+ - fencegate_open.obj
+ - fencegate_closed.obj
 
 License of sounds
 --------------------------------------
@@ -64,3 +70,9 @@ Opening-Sound created by CGEffex (CC BY 3.0), modified by BlockMen
   door_open.ogg
 Closing-Sound created by bennstir (CC BY 3.0)
   door_close.ogg
+fencegate_open.ogg:
+  http://www.freesound.org/people/mhtaylor67/sounds/126041/ - CC0
+fencegate_close.ogg:
+  http://www.freesound.org/people/BarkersPinhead/sounds/274807/ - CC-BY-3.0
+  http://www.freesound.org/people/rivernile7/sounds/249573/ - CC-BY-3.0
+

mods/doors/init.lua

@@ -642,3 +642,62 @@ minetest.register_craft({
 	}
 })
 
+
+----fence gate----
+
+local fence = {
+	description = "Fence Gate",
+	drawtype = "mesh",
+	tiles = {"default_wood.png"},
+	paramtype = "light",
+	paramtype2 = "facedir",
+	sunlight_propagates = true,
+	is_ground_content = false,
+	drop = "doors:gate_closed",
+	connect_sides = { "left", "right" },
+	groups = {snappy = 1, choppy = 2, oddly_breakable_by_hand = 2,
+		flammable = 2, fence = 1},
+	on_rightclick = function(pos, clicker)
+		local node = minetest.get_node(pos)
+		local def = minetest.registered_nodes[node.name]
+		minetest.swap_node(pos, {name = def.gate, param2 = node.param2})
+		minetest.sound_play(def.sound, {pos = pos, gain = 0.3,
+			max_hear_distance = 8})
+	end,
+	selection_box = {
+		type = "fixed",
+		fixed = {-1/2, -1/2, -1/4, 1/2, 1/2, 1/4},
+	},
+}
+
+local fence_closed = table.copy(fence)
+fence_closed.mesh = "doors_fencegate_closed.obj"
+fence_closed.gate = "doors:gate_open"
+fence_closed.sound = "doors_fencegate_open"
+fence_closed.collision_box = {
+	type = "fixed",
+	fixed = {-1/2, -1/2, -1/4, 1/2, 1/2, 1/4},
+}
+
+local fence_open = table.copy(fence)
+fence_open.mesh = "doors_fencegate_open.obj"
+fence_open.gate = "doors:gate_closed"
+fence_open.sound = "doors_fencegate_close"
+fence_open.groups.not_in_creative_inventory = 1
+fence_open.collision_box = {
+	type = "fixed",
+	fixed = {{-1/2, -1/2, -1/4, -3/8, 1/2, 1/4},
+		{-5/8, -3/8, -14/16, -3/8, 3/8, 0}},
+}
+
+minetest.register_node("doors:gate_closed", fence_closed)
+minetest.register_node("doors:gate_open", fence_open)
+
+minetest.register_craft({
+	output = "doors:gate_closed",
+	recipe = {
+		{"default:stick", "group:wood", "default:stick"},
+		{"default:stick", "group:wood", "default:stick"},
+	},
+})
+

mods/doors/models/doors_fencegate_closed.obj

@@ -0,0 +1,106 @@
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+# www.blender.org
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mods/doors/models/doors_fencegate_open.obj

@@ -0,0 +1,112 @@
+# Blender v2.76 (sub 0) OBJ File: 'gate_open.blend'
+# www.blender.org
+mtllib gate_open.mtl
+o Cube_Cube.001
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