Made all leaves draw as cubes in the inventory regardless of whether
they are plantlike or not in the world.  This has the side effect that a
dropped leaf block will look like a flat sprite.  Oh well, no other way
to fix it.

Adjusted the offset of the textures so that leaves place more accurately
in the world (blame this on the engine, it has no option for me to
adjust the precise vertical visual scale independently from the
horizontal scale, so I have to fake it with the textures).