Fixes #2379. The new name was a misnomer because the values really are X11 video drivers (e.g. ‘cirrus’ or ‘nvidia’), not OpenGL implementations. That it's also used to set an OpenGL implementation for kmscon is just confusing overloading.
Fixes #2242 in a different way (cleaner, I hope).
Close #2200. Thanks to @cpages for suggesting and testing this.
Signed-off-by: Shea Levy <shea@shealevy.com>
