Sorry, I guess I didn't spot your question.

GL_VENDOR, GL_RENDERER are used to circumvent the fact that CEMU will change
behavior related to what it detects. In general you do not want it to know
which GPU you have.

That is more than useful with intel IGD, because when Cemu believe he's on
windows with an intel, it display some kind of Picasso remotely related to a
video game, and you couldn't tell which one it is...

You can set any string like "Giovanni GPU" "Giovanni Ltd." and Cemu will
trigger its [generic] behavior. (Check the title bar on the windows)