Make sure, that a windows initial state is `mouse_in_viewport = false`.
This makes sure, that the mouse entered notification is sent when the
mouse hovers a window for the first time.
For embedded Windows, `NOTIFICATION_WM_MOUSE_ENTER` is currently sent
twice in a row. Remove one of the places where it is sent.
`Window::_update_mouse_over()` is the correct one, because there it is
also called for native windows.
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