Returns the single strongest matching compositor operation
        given an overlay. If no matches are found, None is returned.

        The best match is defined as the compositor operation with the
        highest match value as returned by the match_level method.

def strongest_match(cls, overlay, mode, backend=None):
        """
        Returns the single strongest matching compositor operation
        given an overlay. If no matches are found, None is returned.

        The best match is defined as the compositor operation with the
        highest match value as returned by the match_level method.
        """
        match_strength = [(op.match_level(overlay), op) for op in cls.definitions
                          if op.mode == mode and (not op.backends or backend in op.backends)]
        matches = [(match[0], op, match[1]) for (match, op) in match_strength if match is not None]
        if matches == []: return None
        else:             return sorted(matches)[0]