shell: Set the DPMS state if the screensaver fails to launch or dies

The lock hook in desktop-shell only changes the DPMS state the second
time it is called, because during the first time it launches the
screensaver and wakes the compositor again when the screensaver surface
is configured. However, if the screensaver fails to launch, the output
is left in an enabled state, even thought there's no content being
displayed on the screen.

Fix this by disabling the outputs when the screensaver dies if the
shell is still locked.

https://bugs.freedesktop.org/show_bug.cgi?id=60084

src/shell.c

@@ -2116,7 +2116,16 @@ static const struct wl_shell_interface shell_implementation = {
 static void
 handle_screensaver_sigchld(struct weston_process *proc, int status)
 {
+	struct desktop_shell *shell =
+		container_of(proc, struct desktop_shell, screensaver.process);
+	struct weston_output *output;
+
 	proc->pid = 0;
+
+	if (shell->locked)
+		wl_list_for_each(output, &shell->compositor->output_list, link)
+			if (output->set_dpms)
+				output->set_dpms(output, WESTON_DPMS_STANDBY);
 }
 
 static void