Put screen effects render handling inside a function

This de-duplicates the screen effects rendering code by putting it
inside a function, Graphics::renderwithscreeneffects(), and using that
instead of copy-pasted code.

desktop_version/src/Graphics.h

@@ -131,6 +131,7 @@ public:
 	void screenshake();
 
 	void render();
+	void renderwithscreeneffects();
 
 	bool Hitest(SDL_Surface* surface1, point p1, SDL_Surface* surface2, point p2);