server: Make wl_object and wl_resource opaque structs

With the work to add wl_resource accessors and port weston to use them,
we're ready to make wl_resource and wl_object opaque structs.  We keep
wl_buffer in the header for EGL stacks to use, but don't expose it by
default.  In time we'll remove it completely, but for now it provides a
transition paths for code that still uses wl_buffer.

Reviewed-by: Jason Ekstrand<jason@jlekstrand.net>

src/wayland-shm.c

@@ -283,6 +283,9 @@ wl_shm_buffer_create(struct wl_client *client,
 WL_EXPORT struct wl_shm_buffer *
 wl_shm_buffer_get(struct wl_resource *resource)
 {
+	if (resource == NULL)
+		return NULL;
+
 	if (wl_resource_instance_of(resource, &wl_buffer_interface,
 				    &shm_buffer_interface))
 		return wl_resource_get_user_data(resource);