RenderTexture オブジェクトを解放します
This function releases the hardware resources used by the render texture.
The texture itself is not destroyed, and will be automatically
created again when being used.
As with other "native engine object" types, it is important to pay attention to the lifetime of any
render textures and release them when you are finished using them, as they will not be garbage
collected like normal managed types.
関連項目: Create、IsCreated 関数