当Director运行内存不足时，它会卸载一些闲置的演员cast member.
Director可以定义这个卸载的优先级。但这优先级的定义也很诡异。
分为：
3—Normal Sets the selected cast members to be removed from memory after any priority 2 cast members are
removed.
2—Next Sets the selected cast members to be among the first removed from memory.
1—Last Sets the selected cast members to be the last removed from memory.
0—Never Sets the selected cast members to be retained in memory. These cast members are never unloaded.
优先顺序是，2-3-1-0. 囧。
再理解了一下，应该是这样：大部分的优先级都是在提前和退后的中间，另有Never。
序号的问题，就不表了。