several bitmaps in a skin from regions of the same image file.
  Usage is:
  <Bitmap id="body" file="main.bmp" alphacolor="#FF0000">
    <SubBitmap id="sub1" x="0" y="10" width="100" height="100" />
    <SubBitmap id="sub2" x="100" y="10" width="100" height="100" />
  </Bitmap>