Don't initialize GL if it has been disabled

We've observed hangs of mutter when it initializes GTK+, which
are caused by initializing GL, which in turn makes xwayland
call back into mutter. With this change, mutter should just
disable GL support in GDK, and things will work.

gdk/x11/gdkglcontext-x11.c

@@ -586,6 +586,9 @@ gdk_x11_screen_init_gl (GdkScreen *screen)
   if (display_x11->have_glx)
     return TRUE;
 
+  if (_gdk_gl_flags & GDK_GL_DISABLE)
+    return FALSE;
+
   dpy = gdk_x11_display_get_xdisplay (display);
 
   if (!glXQueryExtension (dpy, &error_base, &event_base))