'Recodes the video into a different size and sets its bit rate
to something suitable. In theory we are also doing this to ensure
that the video is cleaned. Maybe.
Will return the location of the temporary file. It is up to the
calling function to remove the temporary file after its completed.
`size`: a tuple of the width and height'

def get_encoded(self, size):

    assert self.is_valid()
    dest = tempfile.mkstemp(suffix='.webm')[1]
    self._call('encode', False, '-s', ('%sx%s' % size), dest)
    return dest
