'getGraphicsControlExt(duration=0.1, dispose=2)
Graphics Control Extension. A sort of header at the start of
each image. Specifies duration and transparancy.
Dispose
* 0 - No disposal specified.
* 1 - Do not dispose. The graphic is to be left in place.
* 2 - Restore to background color. The area used by the graphic
must be restored to the background color.
* 3 - Restore to previous. The decoder is required to restore the
area overwritten by the graphic with what was there prior to
rendering the graphic.
* 4-7 -To be defined.'

def getGraphicsControlExt(self, duration=0.1, dispose=2):

    bb = '!\xf9\x04'
    bb += chr(((dispose & 3) << 2))
    bb += intToBin(int((duration * 100)))
    bb += '\x00'
    bb += '\x00'
    return bb
