Viewports can be set as texutre for Sprite or Sprite3D nodes.
Support 3D & 2D scenes with option: Rendering > Usage in inspector.
.tscn file)ViewPort nodeViewPort node, chose Instatiate child scene, and select the scene to render as a textureColors seems to be washed out (at least on certain instances).
Fix:
Sprite or whatever…), add a new SpatialMaterial from the inspectorSpatialMaterial: Flags > Transparent | Unshaded | Albedo Tex Force srgbSpatialMaterial in a same manner as earlier with: Albedo > Texture