From 1.21.6 vanilla's renderBackground drew its own panorama and blurred
at the vanilla setting, then Saturn's panorama was drawn unblurred on top.
SaturnScreenFabric now draws Saturn's panorama in renderBackground, blurs
it and keeps vanilla's darkening in a world, and GameRendererMixin gives
the blur shader Saturn's strength while a Saturn screen is open. Before
1.21.6 the existing blur reads the same setting.
Co-Authored-By: Claude Opus 5.5 <[email protected]>