Comment 6 for bug 1714459

And here is some X11 code guessing the subpixel order. COGL has it hard-coded on line 329:
  https://git.gnome.org/browse/mutter/tree/cogl/cogl/cogl-xlib-renderer.c#n306

So it looks like a mistake that it works at all on X11. Although I might be looking at the wrong code...