Update screenshake position in fixed-timestep loop

Otherwise the screen will shake too fast for my liking.

Also I'm planning to add an FPS limiting option later (because right
now, un-capping the FPS is pretty wasteful and eats up lots of
resources, especially since I have only a 60hz monitor), and it'd feel
weird if screen shaking updated every delta timestep.

desktop_version/src/Graphics.h

@@ -130,6 +130,10 @@ public:
 
 	void flashlight();
 	void screenshake();
+	void updatescreenshake();
+
+	int screenshake_x;
+	int screenshake_y;
 
 	void render();
 	void renderwithscreeneffects();