>Please try using force_glsl_extensions_warn=true it works around some game bugs.

That generated the two new messages in the terminal...

ATTENTION: default value of option force_glsl_extensions_warn overridden by
environment.
ATTENTION: option value of option force_glsl_extensions_warn ignored.

... so I assume it was picked up properly, but rendering behaviour didn't
change. The nebula clouds were still missing on the loading screen (so space is
just black) and in-game looked the same as the screenshot I originally posted
(screen is just white except for the menu). All in all, no change.

I'll try with your hack patch and see if that changes behaviour.