'Gets a screenshot half way through the video. 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_screenshot(self, size):

    assert self.is_valid()
    assert self.meta.get('duration')
    halfway = int((self.meta['duration'] / 2))
    dest = tempfile.mkstemp(suffix='.png')[1]
    self._call('screenshot', False, '-vframes', '1', '-ss', str(halfway), '-s', ('%sx%s' % size), dest)
    return dest
