render when visible only

src/framework/engine.cpp

@@ -75,12 +75,14 @@ void Engine::run()
         update(ticks - lastFrameTicks);
         lastFrameTicks = ticks;
 
-        // render
+        // render only when visible
+        if(Platform::isWindowVisible()) {
             render();
 
             // swap buffers
             Platform::swapBuffers();
         }
+    }
 
     lastFrameTicks = 0;
     m_stopping = false;