Use PNG instead of JPG

unity-environment/Assets/ML-Agents/Scripts/Batcher.cs

             foreach (Texture2D obs in info.visualObservations)
             {
                 agentInfoProto.VisualObservations.Add(
-                    ByteString.CopyFrom(obs.EncodeToJPG())
+                    ByteString.CopyFrom(obs.EncodeToPNG())
                 );
             }
             return agentInfoProto;