Define `m_winner` if there is no winner or if it is a due game

[EvoLandEco]

m_winner can be set to 0, 1, 2 to refer to each of the players, what if there is no winner, e.g., a round has not finished yet, or the players have the same score so it could be a due game?

I remember when I first implemented the scoring_board class, I set m_winner's default value to -1 to indicate no winner, but we might need to find a more rigorous way to define those two cases mentioned above.

[EvoLandEco]

also consider two players have the same score but the score is higher than the rest, I think randomly generating a winner in such case is a bit unfair though