* No more access to the singleton instance from everywhere (RenderingEngine::get_instance dropped)
* RenderingEngine::get_timer_time is now non static
* RenderingEngine::draw_menu_scene is now non static
* RenderingEngine::draw_scene is now non static
* RenderingEngine::{initialize,finalize} are now non static
* RenderingEngine::run is now non static
* RenderingEngine::getWindowSize now have a static helper. It was mandatory to hide the global get_instance access
