スクリプトからテクスチャのピクセル情報を読み込むかどうか
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.
When texture is not readable, it consumes much less memory, because a system-memory copy does not have
to be kept around after texture is uploaded to the graphics API.