Comment # 22 on bug 98238 from Gregor Münch

(In reply to Shmerl from comment #21)
> It's now in master. It should eventually land in /etc/drirc in distros, but
> until then, build Mesa from source and set this variable when running the
> game:
> 
> glsl_correct_derivatives_after_discard=true

Not needed, Edmondos patch sets this automatically.
https://cgit.freedesktop.org/mesa/mesa/commit/?id=2ea16f08f3f57ce32a2912ddd0ed74027a89a547

If someone wants old behavior back set in game's launch option:
glsl_correct_derivatives_after_discard=false %command%