fixup scenario advancing

the "gamestate().remove_snapshot();" is unneded and would undo the
"gamestate().set_snapshot();" in playcampaign

src/saved_game.hpp

@@ -55,6 +55,7 @@ class saved_game
 
 	config& get_starting_pos();
 	config& replay_start() { return replay_start_; }
+	const config& replay_start() const { return replay_start_; }
 	/**
 	 * If the game is saved mid-level, we have a series of replay steps
 	 * to take the game up to the position it was saved at.

src/savegame.cpp

@@ -653,8 +653,8 @@ void replay_savegame::create_filename()
 }
 
 void replay_savegame::write_game(config_writer &out) {
-	gamestate().remove_snapshot();
 	savegame::write_game(out);
+
 	gamestate().write_carryover(out);
 	out.write_child("replay_start", gamestate().replay_start());
 	out.write_child("replay", gamestate().replay_data);

src/savegame.hpp

@@ -131,7 +131,7 @@ class savegame
 	void set_error_message(const std::string& error_message) { error_message_ = error_message; }
 
 	const std::string& title() { return title_; }
-	saved_game& gamestate() { return gamestate_; }
+	const saved_game& gamestate() { return gamestate_; }
 	config& snapshot() { return snapshot_; }
 
 	/** If there needs to be some data fiddling before saving the game, this is the place to go. */
@@ -161,7 +161,7 @@ class savegame
 	/** Throws game::save_game_failed. */
 	scoped_ostream open_save_game(const std::string &label);
 	friend class save_info;
-
+	//before_save (write replay data) changes this so it cannot be const
 	saved_game& gamestate_;
 
 	/** Gamestate information at the time of saving. Note that this object is needed here, since