Add clutter_x11_set_use_stereo_stage() that can be called
before clutter_init() so that the CoglDisplay we create and all
stages created from that CoglDisplay will be created with a
stereo fbconfig.
This is done in clutter-x11 because of the similarity to the
existing clutter_x11_set_use_argb_visual(), and because it's
not clear without other examples whether the need to have
stereo enabled from before clutter_init() is universal or
somethign specific to GLX.
Cogl required version is increased to 1.20, which has the
required API.
https://bugzilla.gnome.org/show_bug.cgi?id=732706
c32dc34080