Before we can present the buffer we need to wait for the fence to finish. This fixes severe flickering of unfinished rendering in many demos/tests. This has been broken for a while, I think. Note, this is for the non-DRI / Xlib-based GLX. Signed-off-by: Brian Paul <brianp@vmware.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21993>