callback = function (buffer) {
                var data = new Blob([buffer], { type: 'application/octet-stream' });
                // Returns a URL you can use as a href.
                var objUrl = window.URL.createObjectURL(data);
                // Simulates a link to it and click to dowload.
                var a = document.createElement("a");
                document.body.appendChild(a);
                a.style.display = "none";
                a.href = objUrl;
                a.download = "envmap.babylon.hdr";
                a.click();
            }
Saves as a file the data contained in the texture in a binary format.
This can be used to prevent the long loading tie associated with creating the seamless texture as well
as the spherical used in the lighting.
@param url The HDR file url.
@param size The size of the texture data to generate (one of the cubemap face desired width).
@param onError Method called if any error happens during download.
@return The packed binary data.