Use WriteableBitmap to copy pixel data. This does not currently work, probably because the raw data values need to be written, whereas currently PNG data is being written.
Xaml graphics drawing fixes. Graphics draw correctly but currently nothing happens because Invalidate is never called. Need to figure out the right place to call Invalidate.
Finally, some of the Graphics tests are working. Needed to add an Invalidate() in the UpdateTiles() code.