output: reset the pending state on failed commit

It can be surprising for callers to stash pending changes, commit, get a
failure, then set some other pending changes, commit again, and get another
failure because of the previously-pending changes.

Instead, make commit reset the pending state on failure.

types/wlr_output.c

@@ -472,6 +472,7 @@ bool wlr_output_commit(struct wlr_output *output) {
 	wlr_signal_emit_safe(&output->events.precommit, &event);
 
 	if (!output->impl->commit(output)) {
+		output_state_clear(&output->pending);
 		return false;
 	}