So I've had a go at compiling mesa with the fix applied.
I'll list what I've done here to be absolutely clear.

Cloned a new copy of mesa
> git clone https://gitlab.freedesktop.org/mesa/mesa.git
Downloaded the patch file from the link Michel posted into the same directo=
ry
Patched the source file
> patch src/gallium/auxiliary/cso_cache/cso_cache.=
c mesa.patch
Compiled the driver with a handy script:
> https:=
//pastebin.com/nmYaj2az
I override (but don't replace) Debian's drivers with the compiled mesa driv=
ers:
> https:=
//pastebin.com/VX8PUt5W
Rebooted and cleared out my mesa shader cache from ~/.cache and
~/.steam/steam/steamapps/shadercache/

The stability of the game is noticeably improved - I was able to play right
through the first tutorial battle, fight several more battles up to the poi=
nt
where you build Alien Containment. Must've played it for at least an hour.

I've not tested whether Enemy Within any other game is affected by the chan=
ge
however.