data-device: Use weston_surface_has_content()

Now we've got a wrapper which tells us whether or not the surface has
valid content, use it.

The 'XXX' comment was removed because it's the same pattern as every
other surface-role implementor: if the surface is not mapped but does
have valid content, then map it.

Signed-off-by: Daniel Stone <daniels@collabora.com>

libweston/data-device.c

@@ -420,8 +420,7 @@ drag_surface_configure(struct weston_drag *drag,
 	assert((pointer != NULL && touch == NULL) ||
 			(pointer == NULL && touch != NULL));
 
-	/* XXX: Why are we checking for a valid buffer here too ... ? */
-	if (!weston_surface_is_mapped(es) && es->buffer_ref.buffer) {
+	if (!weston_surface_is_mapped(es) && weston_surface_has_content(es)) {
 		if (pointer && pointer->sprite &&
 			weston_view_is_mapped(pointer->sprite))
 			list = &pointer->sprite->layer_link;