When using the Wayland backend this sets a constraint that the
CoglRenderer selects the Wayland EGL winsys.
When a Wayland compositor display is set it now also sets a constraint
that the render should use EGL because only EGL renderers will set up
the required wl_drm global object.
The X11 backend now sets the X11 constraint.
Reviewed-by: Emmanuele Bassi <ebassi@linux.intel.com>
