Move return to lab code to Game::returntolab()

Again, I'll keep gamestate 96 and 97 for compatibility reasons, but
ultimately we shouldn't be using gamestates to return to the lab.

desktop_version/src/Game.cpp

@@ -1461,10 +1461,7 @@ void Game::updatestate()
             if(graphics.fademode == 1)	state++;
             break;
         case 97:
-            gamestate = GAMEMODE;
-            graphics.fademode = 4;
-            startscript = true;
-            newscript="returntolab";
+            returntolab();
             state = 0;
             break;
 
@@ -7354,3 +7351,11 @@ void Game::quittomenu()
     }
     script.hardreset();
 }
+
+void Game::returntolab()
+{
+    gamestate = GAMEMODE;
+    graphics.fademode = 4;
+    startscript = true;
+    newscript="returntolab";
+}

desktop_version/src/Game.h

@@ -380,6 +380,7 @@ public:
     int playgc;
 
     void quittomenu();
+    void returntolab();
 };
 
 extern Game game;