Get mouse position when get_viewport().set_embedding_subwindows(false)

**Your Godot version:**
4.0.2

**Issue description:**
You can not longer get window position using OS.get_window_position(). You also can't rely on global mouse if you have multiple windows. I suggest we update the documentation to tell people to do: Vector2(get_parent().position) + get_local_mouse_position()

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_os.html
https://docs.godotengine.org/en/stable/classes/class_window.html