remove enemyTree (quad tree holding enemies) from game type as this is not static geometry and, since enemies are moving every frame, this should also be regenerated on every frame

fcore/game-update.sml

@@ -2,16 +2,16 @@ structure GameUpdate =
 struct
   fun update (game, input) =
     let
-      val {player, walls, wallTree, platforms, platformTree, enemies, enemyTree} =
-        game
+      val {player, walls, wallTree, platforms, platformTree, enemies} = game
 
       val player = Player.runPhysicsAndInput (game, input)
 
+      val enemyTree = Enemy.generateTree enemies
+
       (* check player-enemy collisions and react *)
       val (player, enemies, enemyTree) =
         PlayerEnemy.checkCollisions
           (player, enemies, enemyTree, #projectiles player)
-
     in
       { player = player
       , walls = walls
@@ -19,7 +19,6 @@ struct
       , platforms = platforms
       , platformTree = platformTree
       , enemies = enemies
-      , enemyTree = enemyTree
       }
     end
 end