Allow entity-map:get-entity to accept a default value if none is found

modules/ces/entity/entity-map.scm

@@ -70,10 +70,12 @@ and integer already. Returns false if passed an invalid entity."
    ((integer? entity) entity)
    (else (error "Invalid entity-key parameter" entity))))
 
-(define (get-entity entity-map entity)
+(define* (get-entity entity-map entity #:key (default #f))
   "Returns the entity with the supplied entity-id if one exists.
 Returns false if no entity exists with that id."
-  (hashtable-ref (entity-map-entities entity-map) (entity-key entity) #f))
+  (hashtable-ref (entity-map-entities entity-map)
+                 (entity-key entity)
+                 default))
 
 (define* (get-entities entity-map #:key (criteria? game-entity?))
   "Returns a list of all entities. When criteria? is provided,