`Viewport.to_screen_rect` is used only in a single location to compare it
to `Rect2i()`.
When called from `SubViewport`, `to_screen_rect` is always equal to `Rect2i()`.
When called from `Window`, `to_screen_rect` is always different from `Rect2i()`.
So the comparison `to_screen_rect != Rect2i()` can be replaced by
`Object::cast_to<Window>(this)`.
This allows the removal of `Viewport.to_screen_rect` and the simplification of
`Viewport::_set_size`.
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