add GameType.game_type which stores player and wall types, add GameUpdate.update function which takes a game type and returns a new game type, and refactor player/wall files, and gl-draw file, in light of these changes

fcore/quad-tree.sml

@@ -2,6 +2,8 @@ signature QUAD_TREE =
 sig
   type t
 
+  val empty: t
+
   datatype collision_side =
     QUERY_ON_LEFT_SIDE
   | QUERY_ON_TOP_SIDE
@@ -41,6 +43,8 @@ struct
       }
   | LEAF of item vector
 
+  val empty = LEAF (Vector.fromList [])
+
   fun fromItem (itemID, startX, startY, width, height) =
     let
       val item = mkItem (itemID, startX, startY, width, height)