Make upcoming piece selection a little more forgiving

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Upcoming piece selection is currently implemented using a piece bag and a random number generator. This approach can result in an unreasonable number of the same pieces being vended in a row. Especially earlier in a game, the upcoming pieces should be weighted in favor of the player being able to place them. Some possible improvements:

• Ban consecutive piece types outright earlier in a game
• When a piece is placed, reduce the number of that piece type in the bag for the next iteration

The current behavior more or less matches Frenzic, but being sabotaged by an RNG is not fun

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@spencerv: I've added a check to prevent the same pie piece from coming up two times in a row (doesn't apply to piece color, that would be annoying) -- when you have some spare time, would love your input on how it feels game play wise