Convert door model to B3D (#2906)

Using a B3D model I worked out that it would be possible to have only two models (instead of the current four) for the doors and still keep all the mirroring correct when the door is open/closed.

Author: An0n3m0us

mods/doors/init.lua

@@ -448,16 +448,16 @@ function doors.register(name, def)
 	def.collision_box = {type = "fixed", fixed = {-1/2,-1/2,-1/2,1/2,3/2,-6/16}}
 	def.use_texture_alpha = "clip"
 
-	def.mesh = "door_a.obj"
+	def.mesh = "door_a.b3d"
 	minetest.register_node(":" .. name .. "_a", table.copy(def))
 
-	def.mesh = "door_b.obj"
+	def.mesh = "door_b.b3d"
 	minetest.register_node(":" .. name .. "_b", table.copy(def))
 
-	def.mesh = "door_a2.obj"
+	def.mesh = "door_b.b3d"
 	minetest.register_node(":" .. name .. "_c", table.copy(def))
 
-	def.mesh = "door_b2.obj"
+	def.mesh = "door_a.b3d"
 	minetest.register_node(":" .. name .. "_d", table.copy(def))
 
 	doors.registered_doors[name .. "_a"] = true