It turns out using a TextureRect node isn't necessary :)
The typical ViewportContainer + Viewport setup can be used just
fine to enable filtering on the ViewportTexture returned by the
Viewport.
The performance of the new method is equivalent to the old one.
This demo shows how to scale the 3D viewport rendering without
affecting 2D elements such as the HUD. It also demonstrates how to
toggle filtering on a viewport by using TextureRect to display the
ViewportTexture delivered by the Viewport node.
Hugo Locurcio
Aaron Franke