New mesh door models, and extensive door API

This patch replaces the default door nodes with a new mesh model
and nodes.

Two new models were added that are 2 blocks high. One for left-hinge
and one for right-hinge doors. This allows us to make a single texture
fit on both models. The alternative would have been 1 model and 2
unmapped textures, which is more work for mod developers.

Doors work exactly like the old doors, including ownership, breaking
doors, opening and closing.

Under the hood, we can prevent the top part of the door from being
obstructed by placing an invisible node. This prevents liquids from
flowing through doors or people placing sand or other blocks in the
top half. The door code automatically places and removes these as
needed.

Metadata is used to store door state, just like the old version.

A doors API is added, it allows mods to use the API to open/close or
toggle door states without worrying about sounds, permissions and
other details. This is intended for e.g. mesecons. This API allows
mods to manipulate or inspect doors for players or for themselves.

In-game old door nodes are automatically converted using an ABM and
preserve ownership and orientation and state.

TNT blows up all doors and trapdoors except for the steel ones,
who can survive a blast. We return an itemstack in on_blast(),
which requires a TNT API patch which is also pending.

We enable backface culling for most of these doors, as this gives
the identical visual appearance that the old doors had. In the case
of the glass door, there's a slight twist.

The texture files used by the new doors have new names that do
not conflict with previous texture file names to avoid texture
pack conflicts.

Thanks to red-001 <red-001@users.noreply.github.com> for some
of the conversion code, cleanups, and extra textures.

game_api.txt

@@ -71,43 +71,55 @@ doors.register_door(name, def)
  -> Registers new door
 
 doors.register_trapdoor(name, def)
-^ name: "Trapdoor name"
+^ name: "mod_door"
 ^ def: See [#Trapdoor definition]
  -> Registers new trapdoor
 
+doors.get(pos)
+^ pos = { x = .., y = .., z = ..}
+ -> Returns an ObjecRef to a door, or nil if the pos did not contain a door
+
+    Methods:
+        :open(player)   -- Open the door object, returns if door was opened
+        :close(player)  -- Close the door object, returns if door was closed
+        :toggle(player) -- Toggle the door state, returns if state was toggled
+        :state()        -- returns the door state, true = open, false = closed
+
+    the "player" parameter can be omitted in all methods. If passed then
+    the usual permission checks will be performed to make sure the player
+    has the permissions needed to open this door. If omitted then no
+    permission checks are performed.
+
 #Door definition
 ----------------
 {
 	description = "Door description",
 	inventory_image = "mod_door_inv.png",
-	groups = {group = 1},
-	tiles_bottom: [Tile definition],
-	^ the tiles of the bottom part of the door {front, side}
-	tiles_top: [Tile definition],
-	^ the tiles of the bottom part of the door {front, side}
-	node_box_bottom = regular nodebox, see [Node boxes], OPTIONAL,
-	node_box_top = regular nodebox, see [Node boxes], OPTIONAL,
-	selection_box_bottom = regular nodebox, see [Node boxes], OPTIONAL,
-	selection_box_top = regular nodebox, see [Node boxes], OPTIONAL,
-	sound_open_door = sound play for open door, OPTIONAL,
-	sound_close_door = sound play for close door, OPTIONAL,
-	only_placer_can_open = true/false,
+	groups = {choppy = 1},
+	tiles = { "mod_door.png" },
+	material = "default:wood", -- used to make a craft recipe
+	sounds = default.node_sound_wood_defaults(), -- optional
+	sound_open = sound play for open door, -- optional
+	sound_close = sound play for close door, -- optional
+	protected = false,
 	^ If true, only placer can open the door (locked for others)
 }
 
 #Trapdoor definition
 ----------------
 {
+	description = "Trapdoor description",
+	inventory_image = "mod_trapdoor_inv.png",
+	groups = {choppy = 1},
 	tile_front = "doors_trapdoor.png",
 	^ the texture for the front and back of the trapdoor
 	tile_side: "doors_trapdoor_side.png",
 	^ the tiles of the four side parts of the trapdoor
-	sound_open = sound to play when opening the trapdoor, OPTIONAL,
-	sound_close = sound to play when closing the trapdoor, OPTIONAL,
-	-> You can add any other node definition properties for minetest.register_node,
-		such as wield_image, inventory_image, sounds, groups, description, ...
-		Only node_box, selection_box, tiles, drop, drawtype, paramtype, paramtype2, on_rightclick
-		will be overwritten by the trapdoor registration function
+	sounds = default.node_sound_wood_defaults(), -- optional
+	sound_open = sound play for open door, -- optional
+	sound_close = sound play for close door, -- optional
+	protected = false,
+	^ If true, only placer can open the door (locked for others)
 }
 
 Fence API