Improved the isometric view

- Added a missing include in player.h
- Fog, transparency and fps display are almost done.
- Elements of the class GraphicConextIso no longer have a load member. It is
now executed in the contructor of the class.
- Improved drawing speed: the map is now visited only once during the
contruction in order to compute a priority list over the tiles.
- Roads are drawn before NPC to enhance the player's immersion.