backend, output: send commit events after applying all in wlr_backend_commit()

We were iterating over involved outputs, applying the new state and
sending the commit event for each one. This resulted in commit
events being fired while we weren't done applying the new state for
all outputs.

Fix this by first applying all of the states, then firing all of
the events.

Closes: https://github.com/swaywm/sway/issues/8829
(cherry picked from commit 7392b3313a)

backend/backend.c

@@ -485,5 +485,10 @@ bool wlr_backend_commit(struct wlr_backend *backend,
 		output_apply_commit(state->output, &state->base);
 	}
 
+	for (size_t i = 0; i < states_len; i++) {
+		const struct wlr_backend_output_state *state = &states[i];
+		output_send_commit_event(state->output, &state->base);
+	}
+
 	return true;
 }