game_config_manger refactoring

This commit prepares a patch to disable unused
addons during a game. We try to reduce codes
that change the game_config config object.

In particular this removes a code that changes
the order of terrain graphics tag. This could
in theory break code that relied on that order
but afaik the terrain_graphics tags offers a
`precedence` that allows users to specify the
order in which terrain graphics are applied.
so no additional compatabiltiy code is needed.

src/game_config_manager.cpp

@@ -274,34 +274,12 @@ void game_config_manager::load_game_config(FORCE_RELOAD_CONFIG force_reload,
 
 		main_transaction.lock();
 
-		// Put the gfx rules aside so that we can prepend the add-on
-		// rules to them.
-		config core_terrain_rules;
-		core_terrain_rules.splice_children(game_config_, "terrain_graphics");
-
 		if (!game_config::no_addons && !cmdline_opts_.noaddons)
 			load_addons_cfg();
 
-		// If multiplayer campaign is being loaded, [scenario] tags should
-		// become [multiplayer] tags and campaign's id should be added to them
-		// to allow to recognize which scenarios belongs to a loaded campaign.
-		if (classification != nullptr) {
-			if (const config& campaign = game_config().find_child("campaign", "id", classification->campaign))
-			{
-				const bool require_campaign = campaign["require_campaign"].to_bool(true);
-				for (config& scenario : game_config_.child_range("scenario"))
-				{
-					scenario["require_scenario"] = require_campaign;
-				}
-			}
-		}
 
 		// Extract the Lua scripts at toplevel.
 		game_lua_kernel::extract_preload_scripts(game_config());
-		game_config_.clear_children("lua");
-
-		// Put the gfx rules back to game config.
-		game_config_.splice_children(core_terrain_rules, "terrain_graphics");
 
 		set_multiplayer_hashes();
 		set_unit_data();

src/saved_game.cpp

@@ -217,12 +217,19 @@ void saved_game::set_defaults()
 {
 	const bool is_loaded_game = this->starting_point_type_ != STARTING_POINT_SCENARIO;
 	const bool is_multiplayer_tag = classification().get_tagname() == "multiplayer";
-
+	const game_config_view& game_config = game_config_manager::get()->game_config();
 	static const std::vector<std::string> team_defaults {
 		"carryover_percentage",
 		"carryover_add",
 	};
 
+
+	if (const config& campaign = game_config.find_child("campaign", "id", classification_.campaign))
+	{
+		bool require_campaign = campaign["require_campaign"].to_bool(true);
+		starting_point_["require_scenario"] = require_campaign;
+	}
+
 	for(config& side : starting_point_.child_range("side")) {
 		// Set save_id default value directly after loading to its default to prevent different default behaviour in
 		// mp_connect code and sp code.