output: fix off-by-one wlr_output_event_present.commit_seq

Backends not supporting presentation feedback call
wlr_output_send_present with a NULL event in their commit handler. Since
the commit hasn't been applied yet, commit_seq still has its old value.
We need to increment it.

An alternative would be to move commit_seq in wlr_output_state. This
would allow to have a pending and a current commit_seq.
wlr_output_send_present could take the pending commit_seq when called
with a NULL event.

types/wlr_output.c

@@ -570,7 +570,7 @@ void wlr_output_send_present(struct wlr_output *output,
 	struct wlr_output_event_present _event = {0};
 	if (event == NULL) {
 		event = &_event;
-		event->commit_seq = output->commit_seq;
+		event->commit_seq = output->commit_seq + 1;
 	}
 
 	event->output = output;