refacto: RenderingEngine is now better hidden

* 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

src/gui/guiEngine.h

@@ -50,6 +50,7 @@ struct image_definition {
 /* forward declarations                                                       */
 /******************************************************************************/
 class GUIEngine;
+class RenderingEngine;
 class MainMenuScripting;
 class Clouds;
 struct MainMenuData;
@@ -150,6 +151,7 @@ class GUIEngine {
 	 */
 	GUIEngine(JoystickController *joystick,
 			gui::IGUIElement *parent,
+			RenderingEngine *rendering_engine,
 			IMenuManager *menumgr,
 			MainMenuData *data,
 			bool &kill);
@@ -188,6 +190,7 @@ class GUIEngine {
 	/** update size of topleftext element */
 	void updateTopLeftTextSize();
 
+	RenderingEngine         *m_rendering_engine = nullptr;
 	/** parent gui element */
 	gui::IGUIElement        *m_parent = nullptr;
 	/** manager to add menus to */