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_win32.cpp

@@ -178,6 +178,7 @@ void    ImGui_ImplWin32_Shutdown()
 
     io.BackendPlatformName = nullptr;
     io.BackendPlatformUserData = nullptr;
+    io.BackendFlags &= ~(ImGuiBackendFlags_HasMouseCursors | ImGuiBackendFlags_HasSetMousePos | ImGuiBackendFlags_HasGamepad);
     IM_DELETE(bd);
 }