要加载的 Mipmap 级别。
Normally, the mipmap streaming system selects which mipmap to load based on the position of all Cameras. Setting this field forces a specific mipmap to be loaded and overrides the mipmap streaming system. The mipmap level can still be modified by budgeting memory. Use the streamingMipmapsPriority property to increase the chance of a particular Texture mipmap level being selected by the mipmap streaming system. The requested level is uneffected by minimumMipmapLevel
另请参阅：ClearRequestedMipmapLevel、minimumMipmapLevel。