ソースコード

using UnityEngine;

public static class TextureExt
{
    public static Texture2D ToTexture2D( this Texture self )
    {
        var sw = self.width;
        var sh = self.height;
        var format = TextureFormat.RGBA32;
        var result = new Texture2D( sw, sh, format, false );
        var currentRT = RenderTexture.active;
        var rt = new RenderTexture( sw, sh, 32 );
        Graphics.Blit( self, rt );
        RenderTexture.active = rt;
        var source = new Rect( 0, 0, rt.width, rt.height );
        result.ReadPixels( source, 0, 0 );
        result.Apply();
        RenderTexture.active = currentRT;
        return result;
    }
}

使用例

using System.IO;
using UnityEditor;
using UnityEngine;

public static class ExampleClass
{
    [MenuItem( "Tools/Example" )]
    private static void Example()
    {
        var type = typeof( GameObject );
        var content = EditorGUIUtility.ObjectContent( null, type );
        var image = content.image;
        var tex = image.ToTexture2D();
        var png = tex.EncodeToPNG();
        File.WriteAllBytes( "result.png", png );
    }
}

参考サイト様