make viewport square regardless of aspect ratio

opengl.cpp

@@ -34,7 +34,14 @@ int main()
                 App.Close();
 
             if (Event.Type == sf::Event::Resized)
+            {
                 glViewport(0, 0, Event.Size.Width, Event.Size.Height);
+                glMatrixMode(GL_PROJECTION);
+                glLoadIdentity();
+                gluPerspective(90.f,
+                        (float)Event.Size.Width / (float)Event.Size.Height,
+                        1.f, 500.f);
+            }
 
         }