graphics: fix handling of elapsed time

graphics.cpp

@@ -37,8 +37,11 @@ int main()
         }
     }
 
+    float last_time = Clock.GetElapsedTime();
     while (App.IsOpened())
     {
+        float ticks = Clock.GetElapsedTime();
+        float elapsed = ticks - last_time;
         sf::Event Event;
         while (App.GetEvent(Event))
         {
@@ -56,12 +59,12 @@ int main()
                 it != shapes.end();
                 it++)
         {
-            it->Rotate(1000 * Clock.GetElapsedTime());
+            it->Rotate(360.0 / 4 * elapsed);
             App.Draw(*it);
         }
 
         App.Display();
-        Clock.Reset();
+        last_time = ticks;
     }
 
     return EXIT_SUCCESS;