This fixes the situation where a `CanvasItem` descendant of a `Viewport` which in turn is a descendant of a `CanvasLayer` prefers the more outer `CanvasLayer` rather than the `Vierport`'s.
Because of that, `CanvasItem`s inside a `Viewport` inside a `CanvasLayer` were being rendered to the main `Viewport` instead of the render target of the innermost one.