TextureImporterFormat 系统为提供的平台选择的格式，如果平台不存在，则选择 TextureImporterFormat.Automatic。
返回将为此平台自动选择的 TextureImporterFormat。
The options for the platform string are "Standalone", "Web", "iPhone", "Android", "WebGL", "Windows Store Apps", "Tizen", "PSP2", "PS4", "PSM", "XboxOne", "Samsung TV", "Nintendo 3DS", "WiiU" and "tvOS".