Backends: clear viewport specific bits in io.BackendFlags on backend Shutdown(). (#6334)

Amend 055e715

backends/imgui_impl_opengl2.cpp

@@ -113,6 +113,7 @@ void    ImGui_ImplOpenGL2_Shutdown()
     ImGui_ImplOpenGL2_DestroyDeviceObjects();
     io.BackendRendererName = nullptr;
     io.BackendRendererUserData = nullptr;
+    io.BackendFlags &= ~ImGuiBackendFlags_RendererHasViewports;
     IM_DELETE(bd);
 }