The code for OpenGL threaded GL dispatch is now finally in Mesa-git, after multiple developers attempts to fix it up. This should improve performance in multiple games for users of the open source Mesa drivers.
Just a note to not get overly excited about threaded OpenGL in Mesa. While it's true it could help a bunch of games, there's also games it likely won't help or even make worse. Feral Interactive's ports already do it, and I'm sure others do too.
For those of you interested in seeing The Witness on Linux, it seems Jonathan Blow mentioned it again recently in a livestream in response to a viewer question. It's hard to argue with what he's saying.