Implement heightmapped world

In order to prevent the player spawing underneath the ground, we place
them as if they were standing upon the highest possible elevation the
terrain could have. On the included static heightmap, this means they
will be floating a large distance above the ground. As you start
walking, you begin to fall, giving an incredibly weird feeling of
shrinking as everything around you seems to get taller. After reaching
the ground, things feel much more natural.