Backends: clear bits set in io.BackendFlags on backend Shutdown(). Clear BackendPlatformName. (#6334, #6335)

Amended with fix for missing clear for ImGuiBackendFlags_HasGamepad.

backends/imgui_impl_opengl3.cpp

@@ -355,6 +355,7 @@ void    ImGui_ImplOpenGL3_Shutdown()
     ImGui_ImplOpenGL3_DestroyDeviceObjects();
     io.BackendRendererName = nullptr;
     io.BackendRendererUserData = nullptr;
+    io.BackendFlags &= ~ImGuiBackendFlags_RendererHasVtxOffset;
     IM_DELETE(bd);
 }