cogl-winsys-egl-wayland: Report an error if we can't find the globals we need

Instead of spinning forever, do a roundtrip, which guarantees that the
global messages have been sent by the time we read the sync message.

If the proper globals aren't initialized yet, error out immediately. This
does mean that users can't use CoglOnscreen with foreign custom surface
types without xdg_shell, but when a use case comes for this, we'll
investigate then...

Reviewed-by: Neil Roberts <neil@linux.intel.com>