Engine: in TMX_LoadPieces, always look-up the piece models.  This fixes a
bug where re-entering the same map would have missing models, presumably
because they got freed somewhere.
Data : checked in 'dungeonTiles.png', which contains the tileset for TMX
format maps (for the Tiled map editor).  I merged yd's existing four sets,
and I added three new tiles for objects (light, player, key).
Data / test TMX map : updated to use the new (combined) tileset, and added
three object layers, also added a player start and two light objects.