gallium/xlib: call fence_finish() in XMesaSwapBuffers()
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