Detect better the hero's ground, including with dynamic entities.

The ground below any point can now be computed (useful for #141, #190
and #191).
The code to determine the ground is simpler and works better.
Fix traversable entities placed over wall tiles.

Now, the ground defined by dynamic entities is correctly taken into
account when detecting the presence of obstacles.

Configurations like a traversable dynamic tile placed above a wall
normal tile now work as expected.
(Before that, the workaround was to make the wall also dynamic.)