make GUI resizable

src/gui/GUI.cc

@@ -22,7 +22,7 @@ namespace jes
                                     SDL_WINDOWPOS_UNDEFINED,
                                     WIDTH,
                                     HEIGHT,
-                                    SDL_WINDOW_OPENGL);
+                                    SDL_WINDOW_OPENGL | SDL_WINDOW_RESIZABLE);
         if (!m_window)
         {
             std::cerr << "Failed to create window!" << std::endl;