Fix crash when a player happens to join the server quicker than the creative inventory filler is called

Fix Lava Bucket in Furnace

making rails selectable again.

Rails weren't selectable, and couldn't be removed from a map asides from
using worldedit (or maybe sand)

Add flipped stair recipe for silly minecrafters and make slabs convert to full cubes when placed on each other

Rename crack.png to crack_anylength.png as required by new engine (needed for proper texture pack compatibility)