纹理数据是否可从脚本读取。
Texture has to be set as "readable" in order for Texture2D.GetPixel, Texture2D.GetPixels and similar functions to work. Textures are not set as readable by default.当纹理不可读取时，它消耗的内存会少很多，因为在将纹理上传到图形 API 之后 便不必保留系统内存副本。