view: Save all buffers associated with view

During the execution of a resize transaction, the buffer associated
with a view's surface is saved and reused until the client acknowledges
the resulting configure event.

However, only one the main buffer of the main surface was stored and
rendered, meaning that subsurfaces disappear during resize.

Iterate over all, store and render buffers from all surfaces in the view
to ensure that correct rendering is preserved.

sway/desktop/output.c

@@ -511,7 +511,7 @@ static bool scan_out_fullscreen_view(struct sway_output *output,
 		return false;
 	}
 
-	if (view->saved_buffer) {
+	if (!wl_list_empty(&view->saved_buffers)) {
 		return false;
 	}