compositor: Downgrade rather than drop buffer reference when copied

When the renderer/backend indicate that they do not need a surface's
buffer content to be preserved, most often because they have copied it,
simply downgrade the buffer reference to 'will not access', rather than
drop the buffer reference altogether.

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

libweston/compositor.c

@@ -2734,7 +2734,7 @@ output_accumulate_damage(struct weston_output *output)
 		 */
 		if (!pnode->surface->keep_buffer) {
 			weston_buffer_reference(&pnode->surface->buffer_ref,
-						NULL,
+						pnode->surface->buffer_ref.buffer,
 						BUFFER_WILL_NOT_BE_ACCESSED);
 			weston_buffer_release_reference(
 				&pnode->surface->buffer_release_ref, NULL);