Fix trapdoor side textures and orientations

Previously, opening a trapdoor caused the side textures to flip.

Fix the incorrect textures.
Also add a texture transform to a tile of the open trapdoor, such
that the closed trapdoor sides use the lower part of the texture
and the open trapdoor sides use the higher part.

Clean up some codestyle issues.

mods/doors/README.txt

@@ -40,8 +40,10 @@ Following textures created by PenguinDad (CC BY-SA 4.0):
 
 Following textures created by sofar (CC-BY-SA-3.0):
   doors_trapdoor_steel.png
-  doors_trapdoor_steel_side.png
+
+Following textures created by paramat (CC-BY-SA-3.0):
   door_trapdoor_side.png
+  doors_trapdoor_steel_side.png
 
 Obsidian door textures by red-001 based on textures by Pilzadam and BlockMen (CC BY-SA 3.0):
   door_obsidian_glass.png