Add a visibility to the dungeon.

For the moment only a box around the player is revealed. Does not
take yet into consideration the walls.
Use now a basic line of sight algorithm for field of vision

With this simple algorithm some walls are not revealed when the
player stays along the wall. Will need to improve this idea.
Improved field of vision algorithm

Field of vision is now based on a circle of a given radius. They were
previously some artifacts if the player stayed too close to a long wall.
This is now fixed. See:
https://sites.google.com/site/jicenospam/visibilitydetermination