UWP - Saving a png as jpg, I want the transparency to go White

Question

I asked a previous question here about saving canvas to png. It works great when I save to png except on problem: the background is transparent, and I want it to be white.

My first idea was to try using JPG encoding and it would automatically add a background in -- I was right! But it added a black background in.

Now unfortunately I don't seem to have control of the production of the canvas, so I can't just add a white background to it. I looked up documentation on canvas and can't just add white to the background of it apparently - if I can just add a white background to a canvas, please tell me, as that will solve my problem.

So the only other solution is to somehow take my PNG-encoded stream from the code below and replace transparency with white. If it can be done inside of a png instead of jpg, I'm fine with that. If it can only be done on a jpg, I'm fine with that too. Any solution to get a white background on this image.

function saveCanvasAsImage(canvasElement) {
    var Imaging = Windows.Graphics.Imaging;
    var picker = new Windows.Storage.Pickers.FileSavePicker();
    picker.suggestedStartLocation = Windows.Storage.Pickers.PickerLocationId.picturesLibrary;
    // picker.fileTypeChoices.insert("JPEG file", [".jpg"]);
    picker.fileTypeChoices.insert("PNG file", [".png"]);
    var fileStream, decoder, encoding, pixels = null;


    var encoderIds = {
        '.png': Imaging.BitmapEncoder.pngEncoderId,
        '.jpg': Imaging.BitmapEncoder.jpegEncoderId
    }

    var encoderId = encoderIds['.jpg'];

    var blob = canvasElement.msToBlob();

    return Imaging.BitmapDecoder.createAsync(Imaging.BitmapDecoder.pngDecoderId,
            blob.msDetachStream())
        .then(function (dc) {
            decoder = dc;

            return picker.pickSaveFileAsync();
        }).then(function (file) {
            if (file) {
                encoderId = encoderIds[file.fileType];
                return file.openAsync(Windows.Storage.FileAccessMode.readWrite);
            } else {
                return WinJS.Promise.wrapError("No file selected");
            }
        }).then(function(stream) {
            fileStream = stream;

            var transform = new Windows.Graphics.Imaging.BitmapTransform();

            return decoder.getPixelDataAsync(
                decoder.bitmapPixelFormat,
                decoder.bitmapAlphaMode,
                transform,
                Windows.Graphics.Imaging.ExifOrientationMode.respectExifOrientation,
                Windows.Graphics.Imaging.ColorManagementMode.colorManageToSRgb
            );

        }).then(function (pixelProvider) {

            pixels = pixelProvider.detachPixelData();

            return Imaging.BitmapEncoder.createAsync(encoderId, fileStream);
        }).then(function (encoder) {
            encoding = decoder;
            //Set the pixel data--assume "encoding" object has options from elsewhere
            encoder.setPixelData(encoding.bitmapPixelFormat, encoding.bitmapAlphaMode,
                encoding.pixelWidth, encoding.pixelHeight, encoding.dpiX, encoding.dpiY,
                pixels);
            //Go do the encoding
            return encoder.flushAsync();
        }).done(function () {
            //Make sure to do this at the end
            fileStream.close();
        }, function () {
            //Empty error handler (do nothing if the user canceled the picker
        });
}