OpenGL calls bound in render method

Post_update call added in class Shape

In fact, we need two update times:
the first one is made to accumulate forces given by the user,
the second one is bound to update the speed and the position
with respect to the accumulated forces

Finally, the update/render order is the following:

• Shape user update, Shape::update
• Shape engine update, Shape::post_update
• Shape rendering, Shape::render

All detailed code relative to the calls is implemented in the scene.cpp file