output: add when field to wlr_output_event_commit

Similar to the one already present in wlr_output_event_precommit.

include/wlr/types/wlr_output.h

@@ -201,6 +201,7 @@ struct wlr_output_event_precommit {
 struct wlr_output_event_commit {
 	struct wlr_output *output;
 	uint32_t committed; // bitmask of enum wlr_output_state_field
+	struct timespec *when;
 };
 
 enum wlr_output_present_flag {

types/wlr_output.c

@@ -607,6 +607,7 @@ bool wlr_output_commit(struct wlr_output *output) {
 	struct wlr_output_event_commit event = {
 		.output = output,
 		.committed = output->pending.committed,
+		.when = &now,
 	};
 	wlr_signal_emit_safe(&output->events.commit, &event);