Add -playassets command-line option

This is used if you're loading a level file from STDIN. The game needs
to know the actual level assets directory you're referring to, since
when it gets the level from STDIN, it doesn't know the actual filename
of the level.

Fixes #309.

desktop_version/src/Game.h

@@ -382,6 +382,7 @@ public:
     int playrx;
     int playry;
     int playgc;
+    std::string playassets;
 
     void quittomenu();
     void returntolab();