gl: add a scary note about initializing thread support for the winsys

gl: add a scary note about initializing thread support for the winsys

We cannot do it as the winsys may crash if we initialize too late.

Example, GLX contexts with Intel drivers:
Intel requires the X Display to be the same in order to share GL
resources across GL contexts.  These GL contexts are generally
accessed from different threads.  Without winsys support it is
nearly impossible to guarentee that concurrent access will not
occur.  This concurrent access could result in crashes or abortion
by the winsys (xcb).

https://bugzilla.gnome.org/show_bug.cgi?id=731525