https://bugs.winehq.org/show_bug.cgi?id=44808
--- Comment #5 from David Oftedal zerooneo@01101001.net --- Thank you very much for that!
I don't have the source code for Mesa ready, but I was able to test the Wine patch, with the latest commit (e1c7a1f7ce03c1e69e008378e90523e85e1c6e8f / wine-3.4-178-ge1c7a1f7ce), and it most definitely resolves the graphics issues with The Witness.