Add missing MP settings for reloaded games. Fixes #21808.

The problam was that mp::configure is not used for reloaded games and so
mp_game_settings will not be fully initialised. This leads to using default
values rather than values set using mp::configure screen or from reloaded game
config.

The optimal solution would be to either display mp::configure for reloaded games
or to populate mp_game_settings using reloaded game config. However, both
solutions require rather a lot of code restructing and are probably not suitable
for a feature freeze.

changelog

@@ -38,6 +38,7 @@ Version 1.11.11+dev:
      when the campaign is reloaded from a non-host side, or after a player rejoins
      from observer status. Hopefully, reloading campaigns is easier after this.
    * Fix bug #21797: "Mandatory WML child missing" when leaving a reloaded game.
+   * Fix bug #21808: Cannot join a reloaded game as an observer.
    * Fixed halos glitching through locations that become shrouded after the
      halo is rendered for the first time.
    * Workaround for bug #18921: disable animations for debug create/kill unit

src/mp_game_utils.cpp

@@ -139,8 +139,15 @@ config initial_level_config(game_display& disp, const mp_game_settings& params,
 	// This will force connecting clients to be using the same version number as us.
 	level["version"] = game_config::version;
 
-	level["observer"] = params.allow_observers;
-	level["shuffle_sides"] = params.shuffle_sides;
+	// If game was reloaded, params won't contain all required information and so we
+	// need to take it from the actual level config.
+	if (params.saved_game) {
+		level["observer"] = level.child("multiplayer")["observer"];
+		level["shuffle_sides"] = level.child("multiplayer")["shuffle_sides"];
+	} else {
+		level["observer"] = params.allow_observers;
+		level["shuffle_sides"] = params.shuffle_sides;
+	}
 
 	if (level["objectives"].empty()) {
 		level["objectives"] = "<big>" + t_string(N_("Victory:"), "wesnoth") +