fix drawing on another window to generate bitmap files on linux

Common/CreateFile.cpp

@@ -152,6 +152,11 @@ static PixelBuffer *GetCompositePixelBuffer(GLenum format, GLenum type)
       (FlGui::instance()->getCurrentOpenglWindow()->getDrawContext());
     newg->show();
     openglWindow::setLastHandled(newg);
+    //waiting for the os to really make the window visible and to call the "draw" function
+    //on (some ?) linux if we do not wait here, the window is not ready
+    //and the picture cannot be generated
+    while(!newg->valid())
+      Fl::wait();
   }
 
   PixelBuffer *buffer;