This splits out the GLeglImageOES define in cogl-egl.h into a private
cogl-egl-private.h header and updates the guards in cogl-egl.h to be
consistent with other top-level headers where we need to be careful
about how __COGL_H_INSIDE__ is defined and undefined, esp when the
gobject introspection scanner is running.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
