Viewport update_mode issue on ViewportContainer ENTER_TREE or VISIBILITY_CHANGED Notification

[butkeim]

Godot version

3.3.stable

System information

Windows 10

Issue description

If a ViewportContainer is visible_in_tree it will set update_mode to all children Viewports to Viewport::UPDATE_ALWAYS when receiving a NOTIFICATION_ENTER_TREE or NOTIFICATION_VISIBILITY_CHANGED.
This will cause performance issues if you are dealing with a lot of ViewportContainer containing a Viewport (in my case for exemple, in the hundreds causing fps drop and CPU over usage).

Steps to reproduce

• Create a ViewportContainer
• Create a sub Viewport (child to the ViewportContainer)
• Duplicate the ViewportContainer hundreds of time, and see the computational impact (using CPU monitoring f. ex).

Minimal reproduction project

No response