Investigate enabling threaded GL for MESA

// This disables threaded rendering on anything using mesa, e.g.
// - nvidia/nouveau
// - amd/gallium
// - intel
// - some software opengl implementations
//
// The client glx vendor string is used to identify those setups as that seems to show the least
// variance between the bad configurations. It's always "Mesa Project and SGI". There are some
// configurations which don't use mesa and which can do threaded rendering (amd and nvidia chips
// with their own proprietary drivers).
//
// This, of course, is very broad and disables threaded rendering on a lot of devices which would
// be able to use it. However, the bugs listed below don't follow any easily recognizable pattern
// and we should rather be safe.
//
// http://cgit.freedesktop.org/xcb/libxcb/commit/?id=be0fe56c3bcad5124dcc6c47a2fad01acd16f71a will
// fix some of the issues. Basically, the proprietary drivers seem to have a way of working around
// a fundamental flaw with multithreaded access to xcb, but mesa doesn't. The blacklist should be
// reevaluated once that patch is released in some version of xcb.
static const char *qglx_threadedgl_blacklist_vendor[] = {
    "Mesa Project and SGI",                // QTCREATORBUG-10875 (crash in creator)
                                           // QTBUG-34492 (flickering in fullscreen)
                                           // QTBUG-38221
    0
};