This simplifies the rendering code and remove all(!) pointers.
There is a number of unresolved issues
* Windows build ?
* position delta computation: migrating to SFML seems to have modified
the frame rate.
* Add margins all around the board for better aestetics.
* Add room for scoring
* Because the texture atlas is a 32x32 grid, we can manipulate
textures as positions on that grid and only create a rectangle
for that texture in the rendering code.
* Avoid hardcoded values in the rendering code.
