Option to change the walking starting command and pg entity used
Implement getter/setters for depth
Merge branch 'master' of git+ssh://dinauz.org/home/pbs/git/bci-interface
DisplayObject can be resized if implemented
Set m_pg to the passed entity
Don't allow fast speed switches
Add backward stimulus to the interpreter
DisplayObject and Background can be implemented with OpenGL

In order to do so, one should:
* Override virtual bool DrawWithGL() to return true
* Write an OpenGL function in Draw/Display function

Background drawing occurs before any other drawing as with non-OpenGL backgrounds
DisplayObject drawing occurs after all SFML objects have been drawn

It is possible to mix SFML drawing and OpenGL drawings, e.g.

void Draw(sf::RenderWindow * app)
{
    /* OpenGL stuff */
    ...

    app->pushGLStates();
    /* SFML stuff */
    ...
    app->popGLStates();

    /* More OpenGL stuff */
    ...
}

However, push/popGLStates() are expensive functions, use sparsely
Merge branch 'master' of git+ssh://dinauz.org/home/pbs/git/bci-interface