add note about Y flip (#2724)

UnitySDK/Assets/ML-Agents/Scripts/Utilities.cs

 
             var t = textureOffset;
             var texturePixels = texture.GetPixels32();
+            // During training, we convert from Texture to PNG before sending to the trainer, which has the
+            // effect of flipping the image. We need another flip here at inference time to match this.
             for (var h = height - 1; h >= 0; h--)
             {
                 for (var w = 0; w < width; w++)