libweston: add opaque backend_id pointer to struct weston_head

As a first step towards heterogeneous outputs, add an opaque pointer
weston_head::backend_id that will be used by backends to identify
their own heads.

See: https://gitlab.freedesktop.org/wayland/weston/-/issues/268

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

include/libweston/libweston.h

@@ -395,6 +395,9 @@ struct weston_head {
 
 	/** Current content protection status */
 	enum weston_hdcp_protection current_protection;
+
+	/** Opaque pointer used by backends to identify heads as theirs */
+	const void *backend_id;
 };
 
 /** Output properties derived from its color characteristics and profile