The One with the Thoughts of Frans

To resolve the weird “ERROR: Missing required OpenGL extensions,” set the environment variable force_s3tc_enable=true.

force_s3tc_enable=true ./start.sh

An alternative, more permanent solution consists of isntalling libtxc-dxtn-s2tc:

sudo apt install libtxc-dxtn-s2tc

Thanks to the game’s forum.