Graphics.Blit(source, dest，mat)：使用着色器拷贝原纹理到RenderTexture
Graphics.Blit通常用于后处理，需要注意的是使用后会修改RenderTexture.active，在此接口后直接使用Graphics绘制接口，不会生效。
public static Texture2D TextureToTexture2D(Texture texture)
{
Texture2D texture2d = new Texture2D(texture.width, texture.height, TextureFormat.RGBA32, true);
RenderTexture rt = new RenderTexture(texture.width, texture.height, 0, RenderTextureFormat.ARGB32);
Graphics.Blit(texture, rt); //使用着色器口拷贝原纹理到RenderTexture
RenderTexture.active = rt;
texture2d.ReadPixels(new Rect(0, 0, rt.width, rt.height), 0, 0);
texture2d.Apply();
return texture2d;
}