Xlib headers define many trivially named objects which can later cause
name collision problems when only cogl.h header is included in a program
or library. Xlib headers are now only included through including the
standalone header cogl-xlib.h.
https://bugzilla.gnome.org/show_bug.cgi?id=661174
Reviewed-by: Robert Bragg <robert@linux.intel.com>
